tree was 4 when it was planted
grows x every year, so it will grow 6x in total

On year 4, tree is 4+4x tall
on year 6, tree is 1.2(4+4x) tall. This is becasue 1/5 is 20%. Since it is 1/5 taller it is the same as multiplying by 1.2
on year 6 it is also 4 + 6x tall

so you'd have the equation
1.2(4+4x) = 4 + 6x
4.8 + 4.8x = 4 + 6x
.8 = 1.2x
.8/1.2 = x
8/12 = x
2/3 = x

there is your answer