The volume of a pyramid is:
[tex] \frac{lwh}{3} [/tex]
You need the height for this. You are given the hypotenuse of a triangle, and the base of that triangle is half of 42 if you can see that. So, let's solve for the height using pythagorean theorem:
[tex]35^2 = 21^2+h^2[/tex]
Solve for h:
[tex]b = \sqrt{35^2-21^2} = 28[/tex]
Now you have all the information, let's just plug everything in:
[tex] \frac{42*42*28}{3} = 597.33[/tex]
So, your volume is 597.33 cm³. Hope everything makes sense. hmy if something doesn't.