Asquare pyramid has a base with side lengths each measuring 40 inches. the pyramid is 21 inches tall, with a slant height of 29 inches

For solving of the area of a square pyramid, we can use the formula below:
A=a²+2a sqrt (a²/4 +h²)
where s for the slant height, r for the a/2, where a is the side length and h for the height
Substitute the values, 
A=40²+(2*40) SQRT (40²/4 +21²)
A=3920
The answer is 3920 for the area of the square pyramid.