first part: put them equal to each other
6x - 25 + 4x = 45 + 3x - 7
10x - 25 = 38 + 3x
7x - 25 = 38
7x = 63
x = 9
second part: plugin
6x - 25 + 4x
6(9) - 25 + 4(9)
54 - 25 + 36
29 + 36
= 65
-- plugging this in one of the equation is enough, since both are the same.
final answer for the second part: 65