What two numbers multiply to get neg 240 and what two same numbers add to get neg 8

Make the number x & y
x*y=-240240
x+y=-8 < you can rearrange this as x=-8-y
Now substitute in this rearrangement
(-8-y)*y=-240
-8y-y² =-240 (add y² and 8y)
0=y²+8y-240 (now factorise the equation)
0=(y+20)(y-12) (for 2 things to multiply and equal zero one or both has to equal zero)
so y+20=0 and/or y-12=0
so y=-20 or y=12
Now substitute these into x+y=-8 to get x
y=-20 then x=12
y=12 then x=-20

Where is the problems