Explanation:
you know how to transform an equation ?
you need to apply the same operating to both sides of the equation. always. otherwise the equality relation is destroyed.
x + 26 = 46
to get to "x = ..." the "+ 26" is in the way.
clearly we need to subtract 26. but we need to do it on both sides.
x + 26 - 26 = 46 - 26
x = 20
that simply means the last tree is 20 in.
it is 26 in shorter than 46 in (fig tree).
that's all there is to it.