if the width is $\displaystyle x$, then the length can be written as $\displaystyle l = 60 - 2x$
and so, $\displaystyle A = x(60 - 2x) = 60x - 2x^2$
for the third question:
solve for first derivative of $\displaystyle A$ and equate it to $\displaystyle 0$ and solve for the value of $\displaystyle x$.. after solving for $\displaystyle x$, substitute it to the $\displaystyle A$ above..