Given the function $f(x) = 5 x^{3/2}$ :

(a) Calculate exactly the value of $y$ when $y = f( 4 )$. Simplify your answer as much as possible.
$y =$ help (numbers)

(b) Calculate exactly the value(s) of $x$ when $f(x) = 6$. If there is more than one such value, enter all possible values as a comma separated list of (exact) values.
$x =$ help (numbers)