When two lines are parallel the alternate exterior angles are equal in measure. So you should set those two expressions equal and solve the equation
5x - 21 = 2x + 12
-2x -2x
3x - 21 = 12
+ 21 + 21
3x = 33
3 3
x = 11
Now substitute 11 into both expressions and since you want the sum of the angles, you add the two measures together!!
5x - 21 + 2x + 12
5(11) - 21 + 2(11) + 12
34 + 34
68 is the answer...... of course both measure are the same because we knew that from the start!!!
Hope you got it!!
