Area of the base = 24^2 = 576 cm^2
Now find the slant height of each side :-
by pythagoras
s^2 = 12^2 + 16^2 = 144 + 256 = 400
s = sqrt 400 = 20 cm
so the area of the 4 triangular sides = 4 1/2 20 * 24 = 960 cm^2
total surface area = 576 + 960 = 1536 cm^2