Question 815894
ax+by=9
(1,-2) and (4,1)
1a-2b=9
4a+b=9

double the second

1a-2b=9
8a+2b=18
add them
9a=27
a=3
3-2b=9
-2b=6
b=-3
ax+by=9
3x-3y=9