/sci/ - Science & Math

[math](x^2 -4x) * (x^2 +2x -3) = 0 [/math]
I'm supposed to solve this using the zero rule. It works well if I say x=0, I think, but I threw it into a calculator and got four answers. Like
[math]x-4=0 [/math] so [math] x = 4 [/math]
[math]x+3=0 [/math] so [math] x = -3 [/math]
[math]x-1=0 [/math] so [math] x = 1 [/math]
I really don't get it.