Fractional Powers Differentiation practice problem

While doing the practice problems for differentiation with fractional powers, I ran across one where I'm given f(x) = 3x^(1/2), and I'm supposed to find f'(4). I followed the rules, getting f'(x) = 3/(2*x^(1/2)). Putting in 4, I get 3/4 as my answer, which is what the problem also said the answer was. Graphing f(x) appears to agree with this result. 

But when I square root, don't I need to add + or - ? Algebraically, it seems I'd end up with plus or minus 3/4. If I didn't have access to a graph, how would I know which is correct? Would I have to just evaluate f(x) for a couple values around 4 and see if it's increasing or decreasing?