I'll start you off..........
Given: f(x) = 1/x
So, f(a+h) = 1/(a+h)
and f(a) = 1/a
So the expression you typed above is just:
[ (1/(a+h) - 1/a] /h
When you simplify that complex fraction above (not hard at all), you get -1/[a(a+h)] as your final answer.
Let's take the numerator........
1/(a+h) - 1/a
To subtract, you get the LCD which is a(a+h),
so
1/(a+h) - 1/a = [a-(a+h)]/a(a+h)
= -h/(a(a+h))
Now, the denominator is just h, so you get
-h/(a(a+h)) divided by h/1 = -1/(a(a+h))