Question

In an isosceles triangle, line segment AC is congruent to line segment AB. Find the value of y in terms of x if one of the remote interior angle is y and the exterior angle is 3x+20.

I think the answer is y=6x-140 but I don't know how they got it.

Answer 1

Y is actually the vertex angle, as one of the remote interior angle.

Answer 2

If I read this ,
y and (3x+20) would be supplementary angles, that is they add up to 180°
y + 3x+20 = 180
y = 160-2x

However, your suggested answer is y = 6x-140, indicating that each of the equal angles could be 3x+20
in that case:
y + 2(3x+20) = 180
y + 6x + 40 = 180
y = 140 - 6x