Input: -13, 20, 0, -4, 11, -1, 17, x output: -15, -11, -9, -2, -1, 5, 5, 13

Answer(s): -5, -1, 0, 2, 11, 17

First, construct two equations.
Then, equal the two equations to each other like this -

x will be the years.

Type A. Type B

3x + 15 = 4x + 10
- 15. - 15

3x = 4x - 5
-4x. -4x

-x = -5
-x/-1 = -5/-1
x = 5

Check:

3(5) + 15 = 4(5) + 10
15 + 15 = 20 + 10
30 = 30

Therefore, the trees will be the same height in 5 years.