Answer
(a) $f(x+y,x-y)=x^2-y^2+3$, (b) $f(xy,3x^2y^3)=3(x^3y^4+1)$
Work Step by Step
(a) Substitute $x+y$ for $x$ and $x-y$ for $y$:
$$f(x+y,x-y)=(x+y)(x-y)+3=x^2-y^2+3$$
(b) Substitute $xy$ for $x$ and $x^2y^3$ for $y$:
$$f(xy,3x^2y^3)=xy\cdot3x^2y^3+3=3(x^3y^4+1)$$
