When you have a polar coordinates like that the numbers are (r,theta)
Where theta is the angle at the origin, and r is distance from the origin.
Simply draw a set of rectangluar axis and draw a line at an angle theta (3[pi]/4).
Then give this line a length of r (4[sqrt]2).
You can then make this line into a right-angled triangle along with the x and y axis, and label the other two sides x and y. You can then get the length of sides x and y with simple trig.
You should get something like rsin(theta) and rcos(theta), but either of them could be negative, or either way around. I'm sure someone else will post it if you are still stuck.