A spider is trying to climb a wall that is 15 metres high. In each hour, it climbs up 3 metres, but falls back 2 metres. In how many hours will it reach the top of the wall?​

the sum of all the angles in a triangle are equal to 180

so you can add   the 3 given angles together to equal 180:

5y + 2y + 2y = 180

or since all three angles have the same letter, combine the 3 angles to equal 180:

9y = 180