Find the value of angle b.

Angle $b$ is an exterior angle of the triangle. The exterior angle of a triangle is equal to the sum of the two opposite interior angles. The two opposite interior angles are $73^\circ$ and $35^\circ$.

$$b = 73^\circ + 35^\circ$$ $$b = 108^\circ$$

Angle $a$ and angle $b$ form a linear pair on a straight line. The sum of angles on a straight line is $180^\circ$.

$$a + b = 180^\circ$$

Substitute the value of $b$ found in Step 1:

$$a + 108^\circ = 180^\circ$$ $$a = 180^\circ - 108^\circ$$ $$a = 72^\circ$$

The angles are:

$$a=72^\circ, b=108^\circ$$