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
the midpoint between
just average them
so given that (3,5) is the midpoint of (-4,5) and (x,y)
so by logic
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