Let's go ahead and find the volume of a prism, then double the dimensions and see what happens to the volume. Then we can apply it to your volume.
volume of a prism with dimensions 2x2x2
v= 2x2x2 = 8 units^3
Now let's double it
volume of prism with dimensions 4x4x4
v= 4x4x4 = 64 units^3
So we see that by doubling the dimensions we multiply the volume by a factor of 8. Let's do that for your volume, 3.5 m^3
3.5 x 8 = 28 m^3