Hi,
So there are two methods to use, elimenation and substitution. I explain elimination in the attachments. It may be the easiest way here.
For substitution, you simply recognise that the equation on top can be rearranged to y = 4x - 6.
Simply substitute the y term in the second equation with the x expression above such that:
3x - 2[4x-6] = 21.
It simplifies to
3x - 8 x = 21 - 12 and therefore
x = (-9/5). The same result as in the elimination method attached.
See the second part for explanation on how to find y and show that the equations intersect.
I hope you can see the second part clearly.