Given the family of functions h(x)=x+cos(ax), where a is a positive constant such that 0<a<4.

For what values of a wiill h(x) have a relative maximum at x=1?

I found the h'(x) first. Then I equated that to zero: h'(x)=1-asin(ax).

Then I'm not sure what to do next, since there are two variables, do I just plug x=1 into the equation then solve for a?