In other words, if you know the equation of the tangent line to a function is y=mx+b, then the value of the derivative of the function at that point x is f'(x) = m, the slope of the tangent line to the function at that point.
In this case the slope of the tangent line to the function at x=2 is -1/4. That means the value of the derivative of the function at x=2 is -1/4. f'(2) = -1/4.