SAT Right triangles Question

$$\mathrm{f}(\mathrm{x})=(\mathrm{x}-\mathrm{a})(\mathrm{x}-\mathrm{b})$$ The function fis defined by the given equation, where a and b are integer constants. If $f(15)>0$ , $f(18)<0$ , and $f(21)>0$ , what is one possible value of $a+b$ ?