Question

If the expression x-3 has a value of -5,then which of the following represents the value of 3x-9

Answer 1

Answer:

-15

Step-by-step explanation:

Italics by itself are comments, italics and underlines are helpful rules, bold highlights little facts.

x-3=-5 : since the expression 'x-3' is being assigned the value '-5'

We then isolate or leave the variable 'x' on one side, usually the left-hand side and move all constants (numbers without letters attached such as 3, 4, 5..etc.) and variables (letters such as x, y, a, b..etc.) that do not have 'x', to the right-hand side of the equality symbol (=). This also includes terms such as 5x, -2y, 50z..etc.

(Remember that the opposite of multiplication is division and vice versa and the opposite of addition is subtraction and vice versa).

To do that, we add 3 to both sides of the equation, because the opposite of subtraction is addition (in this case 3 is being subtracted by x, therefore in order to cancel 3 on the left, we do the opposite of the operation). By doing this, we effectively cancel 3 on the left and bring it to the right-hand side. After all, 3 cannot be in two places at once (well not in this equation).

x=-5+3 which is equal to -2.

Hence, x=-2

Now that we know what 'x' is we can work on the other expression: 3x-9

Substitute the value for 'x' (-2 is what we got) into the 'x' in the expression.

3(-2)-9=?

(Remember that a positive number multiplied by a negative number will ALWAYS be a negative number. In other words, (+) × (-) = (-). Yes, this also includes a negative number multiplied by a positive number).

Therefore, 3×(-2) will equal -6.

-6-9=-15

Hence, 3x-9=-15.

To make sure your answer is correct, do the equation backwards. Add 9 to -15 which would then give you -6.

Here's how that looks: 3x-9 (+9)= -15 (+9).

You would then be left with 3x=-6.

If you remembered the rule typed previously, I said in order to isolate 'x' we do the opposite of the operation. 3x means '3' multiplied by 'x', therefore the opposite of multiplication is division:

3x(÷3)=-6(÷3) ----> 3 divided by 3 equals 1. Anything multiplied by 1 will be equal to itself (x*1= x)

(An asterisk can also symbolise multiplication).

Therefore when 3x is divided by 3, the constants would cancel each other out, 1 would be left looking like this: 1x=... and 1x is the same as x by itself.

-6 divided by 3 is equal to -2 (a positive divided by a negative will always be negative and vice versa).

Hence, x=-2.