how to do this

Question

x+ 4y =-9

-2x -5y =12

Answer 1

There are various methods to do this problem, the common two are substitution or elimination.  Substitution is where you will set one equal to either x or y then put that into the second equation in place of x or y.  Elimination gets rid of x or y by adding or subtracting both equations.  I will show you elimination.

Call "x+4y=-9" 1'

Call "-2x-5y=12" 2'

3' is 2*1'  (thats 2 times equation 1) --> 2x+8y=-18

then add 3' and 2' together, this eliminates x.

3y=-6 so y=-3

then put y into equation 1'

x+ (4*-3) = -9

x - 12 = -9

x = 3

 

Answer         x=3   y=-3