well, x-intercept means for a graphed funciton where the x-axis gets touched/intercepted, Check the picture below, when that occurs, y = 0, so if we want to know what "x" is at that point we can simply set y = 0 and solve for "x".
[tex]-3x-5y=12\implies \stackrel{\textit{setting y = 0}}{-3x-5(0) =12}\implies -3x-0=12 \\\\\\ -3x=12\implies x=\cfrac{12}{-3}\implies x = -4 \\\\[-0.35em] ~\dotfill\\\\ ~\hfill \stackrel{\textit{coordinates}}{(-4~~,~~0)}~\hfill[/tex]
