The sum of angles in a triangle adds up to 180 degrees. So we can subtract 60 to get 120 = 5x + 2y. Now we need to solve for y. Y is on the same side of the equation as x, so we need to move the x by subtracting 5x. Then we have 2y = 120 - 5x. Last, we need to divide by 2, as that is the coefficient of y. Now, we have y = 60 - 2.5x.
This triangle is isosceles with AB = AC ....so....the angles opposite these sides are also =
So angle (5x)° = 40...so these two angles sum to 80°
So...this means that 2y + 20 must = 100
2y = 80
So....y = 40