Me who ever gets it right will be marked as the brainliest.

The answer is 16.
Explaination-
In the right triangle
(Altitude)^2+(base)^2=(hypotenuse)^2
(15)^2+(base)^2=(17)^2
225+(base)^2=289
(Base)^2=289-225
(Base)^2=64
Therefore Base=8 units
Since it is a square pyramid, (base of right triangle)=1/2(one side of pyramid’s base)
8*2=(lenght of one side base)