Perimeter = 420 = 2x + 2y. Therefore y = 210 - x.

Area = xy = x (210 - x).

A = x(210 - x) is a parabola. Use what you've been taught to find it's turning point.

b + h = 15 => b = 15 - h.

A = bh/2 = (15 - h)h/2.

A = (15 - h)h/2 is a parabola. Use what you've been taught to find it's turning point.