Math Problem, Lim H->0 for f(x) = 1/x+3?

So im reviewing for a calc 1 test tomorrow and this problem has got me stumped it seem I don't understand rationalizing correctly. We are supposed to use the limit definition (x+h) for this problem, so I have (1 / (x+h) + 3) - (1/x+3) / h , this is where im stumped im not sure how to rationalize these two fractions, the answer given to the problem is 1/(x+3)^2 if that helps.