= Homework: Section 15.4 Assignment For the following set of variables, find all the relevant second derivatives. f(x,y) = 3x y, where x = s + 2t and y = 2s - t for = 36s + 42t (Type an expression using s and t as the variables.) fst = 24t + 42s (Type an expression using s and t as the variables.) fit = (Type an expression using s and t as the variables.)