Plot the point (-6,7)



Draw the radius vector, r, from the origin (0,0) to the point (-6,7). 



Indicate the angle.  I'll call it .



Draw a vertical line perpendicular down to the x-axis, and
label it .



Draw a horizontal line from the origin to the bottom of
that line, completing a right triangle.  Label it 



Now we find the length of  by the Pythagorean theorem:







So we put  for : 



Now since







Edwin