Will mark brainlest

in the figure, angle a is an exterior angle to triangle bcd.

A) by angle sum property, the sum of 3 angles of a triangle is 180°
therefore sum of angles b,c,d = 180°
b+ c+ d = 180°
b+ 65+55 =180°
b = 180- 120
b= 60°
now angle a+ b= 180°
so angle a= 180 - 60= 120°
so angle a= 120° ,and also sum of the two nonadjusent interior angles of a∆

b)it is proved from above equation the measure of angle a is 120° and measure of angle b is 60°