You can first see that there is a 90° angle which is opposite to ( the vertical angle of) angle 3 which gives you that angle 3 is also 90°. You can then see the vertical angles of angle 2 and the angle of 77°. Therefore angle 2 is 77°. You can then infer that angle 1 + angle 2 is 90° in which case you can subtract angle 2 (77°) from 90° which leaves you with 13° which is the measure of angle a.