Mission dimensions: Impossible!
Do you know the formula for the volume of a pyramid? What dimension are you lacking to find the volume?
The missing dimension is one side of the base triangle. base triangle is a right triangle with one side 4units and hypotenuse 5 units, so the third side is 3 units. Pythagorean triplets.
Now volume of the pyramid = 1/3 x area of base x height.
1/3 x ( 1/2 x 3 x 4 ) x 6= 1/3 x 6 x 6=12 cu units