for f(x)= (3-5x)^1/2,

find f(x+h)-f(x)/h

You probably mean

[f(x+h)-f(x)]/h .

Just plug in the numbers.
[(3-5x-5h)^1/2 -(3-5x)^1/2)]/h

The harder part is taking the limit as h -> 0, which is the derivative of the function.