It looks like your question is incomplete. However, I understand that you are trying to find the equation of the tangent line to a function $f(x)$ at the point $(4,2)$.

To proceed, I need:

1. The function $f(x)$.
2. Confirmation that $a = 4$ (which seems to be given).

Once I have $f(x)$, I can compute the derivative $f'(x)$, evaluate $f'(4)$ to get the slope, and use the point-slope form:

$y - y_1 = m (x - x_1)$

where $(x_1, y_1) = (4,2)$ and $m = f'(4)$.

Please provide the function $f(x)$, and I will complete the solution.

Would you like a step-by-step explanation of derivatives as well?

Related Questions:

1. How do you find the derivative of a function using limits?
2. What is the geometric interpretation of a derivative?
3. How do you find the equation of a normal line to a curve?
4. Can a function have more than one tangent line at a point?
5. What happens if a function is not differentiable at a point?

Tip: The derivative of a function at a point gives the slope of the tangent line, but not all functions are differentiable everywhere!