Question 479943
Assuming PR is a line, and that Q is the midpoint, we can say that PR = PQ +QR

However, since Q is the midpoint this implies PQ = QR. So we have PR = 2PQ

Then we know PR is 42 units, so then 42 = PQ+QR. Then, we know PQ=3y, so we can rewrite everything as:

42 = 3y+QR, but remember PQ and QR are the same, so we have 42 = 3y + 3y, from this we get that y is 7.

PQ = 3y = 3(7) = 21 which makes sense since 2 of these PQ's should add up to 42, which they do