Question

Check whether (5,-1) is a solution of the simultaneous equation x-2y=7

Answer 1

(5,-1)
x-2y=7----(1)


take 5 as x

5-2y=7
y=7-5/-2
y=2/-2
y=-1


now take -1 as x
-1-2y=7
y=7+1/-2
y=8/-2
y=-4

Answer 2

According to your question

given x=5 and y=-1

then, x-2y=7

5-(2*-1)=7

5+2=7

7=7