what is the root for y=(x+3)(x-1)

Re:  what is the root for y=(x+3)(x-1) 

The roots are the x intercepts and the equation of the x axis is y=0

So you need to solve y=0 and y=Re: what is the root for y=(x+3)(x-1)   simultaneously.

that is, you must solve 0=(x+3)(x-1)

If 2 things multiply together to give 0 then one or both of them must be 0

so the only possible solutions are 

x+3=0   and   x-1=0

so x=-3  and    x=1   are the roots.