The given function is
[tex]f(x)=x^2-x-12[/tex]To solve it by factoring, we have to look for two numbers whose product is 12 and whose difference is 1. Those numbers are 4 and 3.
[tex]f(x)=(x-4)(x+3)=0[/tex]Then, we use the zero property
[tex]\begin{gathered} x-4=0\rightarrow x=4 \\ x+3=\rightarrow x=-3 \end{gathered}[/tex]