You can just switch the numbers in front of x and y and make one of them negative. i.e. instead of the original equation 3x+1y=5 write 1x-3y=... Then for the Right hand side of the equation, write the same thing as the Left hand side but replace the x with -9 and y with 3 to get 1x-3y=1(-9)-3(3)=-9-9=-18. That is x-3y=-18 is the answer.