The surface area formula is A+1/2ps
Since the area is 533, we can put 533 = A + 1/2ps and plug everything else in.
A= the area of the base
p= perimeter of the base
s= slant height, which we are trying to find
533 = (13)(13) + 1/2 (2(13)+2(13)) (s)
Now we solve for s.
533= 169 + 26s
-169 -169
364 = 26s
s = 14
Therefore the slant height would be 14. Hope this helps!