What do you mean doubling? You can add 7+7+7 which equals 21.
Answer: 12+8.2n
Step-by-step explanation: Your basically adding 12 to 8.2 times n.
The answer is 9
You divide by 1215 by 15 and the answer is 81
Then you find the square root of 81 and the answer is 9
I am doing with Question Number 2
Let the two numbers be 'x' and 'y'.
According to the question, the product of two numbers is 14.

So, xy=14 (Equation 1)
Now, one of the numbers is 3 ½ times the other.
So, 
(Equation 2)
Substituting the value of 'x' from equation 2 in equation 1, we get



So, y= 2 and -2
So,
and 
x=7 and -7
Numbers are 2,7 and -2,-7.
Sum of numbers 2 and 7 = 9
Sum of numbers -2 and -7 = -9.
Answer:
If we are working in a coordinate plane where the endpoints has the coordinates (x1,y1) and (x2,y2) then the midpoint coordinates is found by using the following formula:
midpoint=(x1+x22,y1+y22)
Step-by-step explanation: