Graph z = 3cos315 + 3isin315 in the complex plane. How can I explain where I would find this equation on a complex plane?

You can just calculate it out to find the coordinates:

3 cos 315 = 2.12

3 i sin 315 = -2.12 i

plot    2.12 - 2.12 i     on the complex plane....

cos 315°   =   45° reference angle in the 4th quadrant =  √2 /2

And sin 315°    is the negative of this

So

z = (3) cos 315°  + ( 3i ) sin 315°  =

[   (3/2)√2  ,  - (3/2) √2 i  ]

Here is the graph