Current Attempt in Progress

Find a number $a$ so that the change of coordinates $s=x+ay,t=y$ transforms the integral ${}_{R}dxdy,$ where $R$ is a parallelogram that has vertices $(0,0),(17,0),(-24,3),(-7,3)$ in the $xy-$plane, into an integral

${}_T|\frac{\text{d}\text{e}\text{l}(x,y)}{\text{d}\text{e}\text{l}(s,t)}|dsdt$

over a rectangle $T$ in the st$-$plane.

Enter an exact answer.

$a=$

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