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

Easy peasy

the midpoint between and is

just average them

so given that (3,5) is the midpoint of (-4,5) and (x,y)

so by logic
and
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