Notice the right angle. Perpendicular to it on the right, we have two complementary angles, 25, and 3x+2 (you can ignore the parentheses basically). Because they are complementary angles, they will add up to 90, so we can make an equation to solve for x:
25+3x+2=90
3x+2=65
3x=63
x=21
So x=21 is the correct answer. Let me know if you have any questions about my explanation.