The first part of the expression 4n is because n is the amount of almond trees and to find the walnut trees, you need to multiply the number of almond trees (n) by for (4n). you would also add 15 because it says he has 15 more than 4 times as many walnut trees.
so here's the breakdown of the sentence:
Daquan has 15 more than (15 +) 4 times as many (4n) walnut trees in his orchard as he has almond trees.
