Ok, so the set up would be 3x+8 = 5x-40. If you solve for x you can then enter it into the equation and get your answer.
So subtract 3x from 3x and 5x. That leaves you with 8 = 2x-40
Next, you add 40 to both sides leaving you with 48 = 2x
Now divide 2 from x and 48 which give x = 24
Enter the value of x into the equation 3x+8
3(24)+8
80
And because we know it is an acute angle, angle ABC = 80 degrees
