Marco and his dad went strawberry picking. Together they collected strawberries that weighed w pounds. On the way back Marco ' dad lost x pounds of strawberries. Marco's strawberries now weighed y pounds. How much did his dad's strawberries weigh now?
Let's say that Marco and his dad picked up w pounds of strawberries together. Then dad lost x pounds of strawberries on the way back, which means that Marco's strawberries now weigh w - x pounds. The question is, how many pounds of strawberries did dad have left? Well, this is exactly what we need to find out, so let's call it y.

Now we have the equation:

w - x = y

which can be rewritten as:

y = w - x