A function f is defined for all real numbers and satisfies

f(2+x)=f(2-x) and f(7+x)=f(7-x)

for all real x. if x=0 is a root of f(x)=0,what is the least number of roots of f(x)=0 must have in the interval -1000$$\leq$$x$$\leq$$1000?

(第二届AIME1984 第十二题)