I want to Learn Finite Element Method. What are the Math Subjects that are prerequisite to FEM?

I am a mechanical engineer. FEM was not taught to us in College. :(

I am profficient in Engineering Mechanics, Differential Calculus, Integral Calculus and Basics of Differential Eequations. I also know Matrix manipulations. I understand Fourier Series, Fast Fourier Transform and Basic of Laplace Transform. In the past I have knowledge in Runge-Kutta, Stokes Theorem, Gradient, Divergence and Curl. I can derive the Navier-Stokes Equation.

What other math subject do I need to learn before I study Finite Element Method? Can you suggest online course for FEM (other than MIT OCW)?

Thank you very much.
Joseph