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 <em>t</em>. We have
(3<em>r</em> + 2<em>s</em> + <em>t</em> + 2<em>u</em> + 3<em>v</em>) - (3<em>r</em> + 2<em>s</em> + 3<em>t</em> + 2<em>u</em> + 3<em>v</em>) = 7 - 17
-2<em>t</em> = -10
<em>t</em> = 5
