Can some1 explain to me what second order partial derivative is and how to find it?

Assuming that z is a function of x and y, the first partial of z with respect to x means to take the derivative of z treating x as the variable, and y as a constant. The second partial, means to do the same thing to the first partial with respect to x. Same for second partial with respect to y; just treat as a constant, and take the derivative twice. You should know that there are also "mixed" partials, where, say, you first take the derivative with respect to x, then with respect to y - or the other way round. Clairaut's theorem says that for most nice functions you will end up with the same answer, no matter which order you choose for the mixed partials.