I need help with this, mainly because it deals with multiple variables.

f(x) = x^2 + 2

What is the equation of the line tangent to f(x) at point P(a, f(a)), where 0 < a < 4 ?

What is the expression for A(x) in terms of a, where A(x) is the area of the triangle formed by the previous tangent line, the x axis, and the line joining P at (4, 0)?

What is the value of a that makes A(x) a minimum and why?