Linear functions SAT question

The correct answer is $-12$. It's given that the functions $f$ and $g$ are defined as $f(x)=\frac{1}{4}x-9$ and $g(x)=\frac{3}{4}x+21$. If the function $h$ is defined as $h(x)=f(x)+g(x)$, then substituting $\frac{1}{4}x-9$ for $f(x)$ and $\frac{3}{4}x+21$ for $g(x)$ in this function yields $h(x)=\frac{1}{4}x-9+\frac{3}{4}x+21$. This can be rewritten as $h(x)=\frac{4}{4}x+12$, or $h(x)=x+12$. The x-intercept of a graph in the xy-plane is the point on the graph where $y=0$. The equation representing the graph of $y=h(x)$ is $y=x+12$. Substituting $0$ for $y$ in this equation yields $0=x+12$. Subtracting $12$ from both sides of this equation yields $-12=x$, or $x=-12$. Therefore, the x-coordinate of the x-intercept of the graph of $y=h(x)$ in the xy-plane is $-12$.