Suppose you have the graphs of f(x) and g(x) and want to find the derivative of f(x)g(x) at x = 2. What you do is use the product rule for differentiation to get
f'(x)g(x) + f(x)g'(x), where f'(x) and g'(x) are the derivatives of f(x) and g(x), respectively.
Then using the graph you can get the values of f(x) and g(x) at x = 2. To get the derivative at x = 2, you look at the slope of the graph at that point on the graph and substitute those values into the equation.