Since triangle ABC is an equilateral triangle, angle 1 is 60°.
Angles 1 and 3 are vertical angles.
Vertical angles have the same measure.
So, the measure of angle 3 is 60°.

Angles 1 and 2 are adjacent angles which form a straight angle.
So, the sum of the measures of angles 1 and 2 is 180°.
Set up an equation and solve for the measure of angle 2.

measure of angle 1 + measure of angle 2	=	180°
60° + measure of angle 2	=	180°
measure of angle 2	=	180° - 60°
measure of angle 2	=	120°

So, the measure of angle 2 is 120°.