Complimentary angles add to 90 degrees. So if you have an angle, x, and another angle, x - 42, they should add together to equal 90...
x + x-42 = 90
2x - 42 = 90
2x = 132
x = 66 degrees (1st angle)
66 - 42 = 24 degrees (2nd angle)
*Check 66 + 24 = 90 :)