f(x) = x^2 + 3*x  function definition
f(y) = y^2 + 3*y
f(w) = w^2 + 3*w
u = 7
g(w) = u^2 + 3*u   (a constant function)


y = f(2)  function application (function call) (calling the function) (applying the function)
w = f(y)
x = f(w)
u = f(x)

print( f(3) );