Make note of the coefficients in the first and fourth equations. They've been conveniently picked so that subtracting one equation from the other eliminates every variable but t. We have
(3r + 2s + t + 2u + 3v) - (3r + 2s + 3t + 2u + 3v) = 7 - 17
-2t = -10
t = 5
