2024年5月北美B卷 · 第 21 题

\( f\left(x\right)=\left(x-a\right)\left(x-b\right) \) 

The function  f is defined by the given equation, where a and b are integer constants. If  f \( (18)>0 \) ,  f \( (21)<0 \) , and  f  \( (24)>0 \) , which of the following could be the value of a+b?