Explanation:
you know how equations are transformed ?
by applying the same operation on both sides of the equation (so that the equality remains).
x + 38 = 79
to get "x = ...", we need to eliminate the "+ 38" in the x side.
clearly we need to subtract 38.
again, whatever e do, we have to do on both sides, otherwise the equality relation is destroyed.
so,
x + 38 - 38 = 79 - 38
x = 41
that is the height of the avocado tree : 41 in.
it is 38 in shorter than 79 in (the fig tree).
that's all that is to this. it is a really simple method.
the same applies to any operation you want to do (+, -, ×, ÷, ...).