In δabc, ∠a = 120° and ∠b = 40°. what is the measure of ∠ c? which side is the longest?

Because the sum of the interior angles of a triangle are equal to 180 degrees, add 120 and 40 to get 160, then subtract from 180. Your answer would be <B = 20 degrees. 

To find out which side is the longest, draw a triangle with angles that are close to these angle measurements. The side across from <A (the largest angle) is the longest side. Depending on your drawing, the longest side would either be BC or CB, depending on your letter order. 

Hope this helps!! :) 

Is this Spanish, because I don't really know Spanish.