Basically what you need to do is add up the expressions, 7x and 3x and make it equal to 90 degrees. Then solve for x. Substitute the value of x for each expression to determine the values of each angle.
7x + 3x = 90
10x = 90
X = 90/10
X = 9.
Angle 1 = 7(9) = 63 degrees
Angle 2 = 3(9) = 27 degrees.
Check
63 + 27 = 90
90 = 90.