Question

Line AB has endpoints A(-4,5) and B(3,5). What is the x-coordinate of a point C such that B is the midpoint of line AC

Answer 1

Easy peasy

the midpoint between $(x_1,y_1)$ and $(x_2,y_2)$ is
$(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2})$
just average them

so given that (3,5) is the midpoint of (-4,5) and (x,y)
$(\frac{-4+x}{2},\frac{5+y}{2})=(3,5)$
so by logic
$\frac{-4+x}{2}=3$ and $\frac{5+y}{2}=5$
times both sides by 2 for everybody
-4+x=6 and 5+y=10
add 4 to both sides for left one and minus 5 from both sides for right
x=10 and y=5

the coordinate of point C is (10,5)
the x coordinate is 10