Answer:
x=43
Explanation:
complementary angles will add up to equal 90 so we set up an equation to solve x
12+x+25=90
step 1 combine like terms 
12+25=37 so now its 
37+x=90
step 2 we subtract each side by 37
90-37=43 
x=43
hope this helps and have a great day :)