quick question about cross product

In these cases the third components are zero (implicitly).  The vectors in full are:

p= -2i -3j+0k and q= 4i +7j+0k

so the formula would be 

(-3)(0) - (0)(7)

It depends what you mean by "don't have".  If you mean these are zero then a2b3 - a3b2 is zero.  If they are just unknown, you can't give that term a value.  

yea, they aren't given. so does that mean i only do the one that applies? a1b2 - a2b1?

Edit*** so it would be?
 a x b= -2 instead of a x b = 0, 0, -2?

No. A 3d vector must always have 3 components.  You just have to leave it in terms of unknown values when you don't have them.  For example, if a2 = -3 and b2 = 7, the third component must be written as -3b3-7a3.

Might be able to give a more satisfactory answer if I saw the question you are trying to answer in its entirety.

find the cross product of vectors p= -2i -3j and q= 4i +7j

Hi Alan and Strider,

Alan, I am glad you understood what Strider was talking about because I had no idea.