What is x-3x-4 but the x is squared to the second power

Hey!  First of all, you can express x squared as x^2.

Secondly, the problem!  I'm assuming you're asking for the values of x - if you're asking what the equation is, it's a quadratic equation, or a second-degree polynomial.  Solving for x, this equation can be factored as (x-4)(x+1)=0.  Because either of these two terms being zero will make the whole equation zero, we can have x-4=0, in which case x=4, or x+1=0, in which case x=-1.  Therefore, x is either 4 or -1