Let's draw an angle of -40°.  Negative angles rotate clockwise from the
right side of the x-axis:



"Coterminal" means "ends up at the same place."

Let's indicate with a green arc the smallest positive angle that
starts at the right hand side of the x-axis and rotates 
counter-clockwise and ends up at the same line that -40°
ends up at. 

The angle we need will be measured by the green counter-clockwise
arc below:



The amount of rotation done by the -40° is 40°

So the amount of counter-clockwise rotation that will be needed
for the angle indicated by the green arc is 360°-40° or 320°.

Answer: 320°

Edwin