Nonlinear functions SAT question

The correct answer is $-324$. It's given that the function $f$ is defined by $f(x)=a^x+b$, where $a$ and $b$ are constants. It's also given that the graph of $y=f(x)$ has a y-intercept at $(0,-323)$. It follows that $f(0)=-323$. Substituting $0$ for $x$ and $-323$ for $f(x)$ in $f(x)=a^x+b$ yields $-323=a^0+b$, or $-323=1+b$. Subtracting $1$ from each side of this equation yields $-324=b$. Therefore, the value of $b$ is $-324$.