Micah did not explain the last step correctly. You cannot cross out a term from the numerator and denominator unless it is a factor. In other words, x² needed to be multiplied and not added in order to cross it out.
Answer:
I think it's like the top answer is 6 the bottom 36.
Let f be the height of the fir trees
Let p be the height of the pine trees
We know from the problem that
f + p = 21 and 4f = 24 + p
rearrange the 2nd equation to get p = 4f - 24 and substitute into the other equation
so...
f + 4f - 24 = 21
5f - 24 = 21
5f = 45
f = 9 - height of the fir tree
substitute back into the 1st equation
9 + p = 21
p = 12 - height of the pine tree