Have you learned matrices yet? I'm going to use that to solve these, please refer to the photos.
I solved the system by using a matrix calculation. The work always needs to be shown and you do that by setting up the matrix like in the first photo and also writing out all your equations. In a TI calculator, to do this you press 2ND-X^-1,GO TO EDIT - [A] - 3×4- now enter the coefficient of the system of equations variables. NOW 2ND MODE- 2ND-X^-1 -GO TO MATH- ALPHA APPS
Now, if you don't know this you may be confuse so after rref([A]) was enter a matrix came up with [1 0 0 -21] in the top row this means x equal -21. So, the next row is y and it came out as [0 1 0 46] so y equals 46 and I'm going to let you figure out what z is by looking at the matrix.
SO... X=-21 Y=46 and Z=-10
Let's Test it
The equation for monthly payment is b = 49.05M.
<u>SOLUTION:</u>
Given, a cellular family phone plan cost $49 per minute plus five cents per minute of long distance service. We have to write an equation for the monthly payment b when M minutes of long distance service are used.
Now, given that,

<em>Equation:</em>
An equation is a statement that the values of two mathematical expressions are equal <em>(indicated by the sign =)</em>
The answer is D since is the only one that has the correct y intercept
1. Find the unit rate
15 books divided by 5 days = 3 books sold per day.
Since we are looking for how many days it would take the store to sell 21 bucks, we could do 21 (amount of books) divided by 3 (how much books is sold in a day).
Therefore, it will take a week / 7 days to sell 21 books.
12*2+7(x) you need to solve for x