Triangle angle-sum theorem
Find m<1

123 Degrees.

Explanation:

This is basically the Exterior Angle Theorem which basically states that the exterior angle (m<1) is equal to the sum of the 2  opposite interior angles. (60 & 63).

Following this theorem your equation should look something like this -

Exterior Angle = Opposite Interior Angle + Opposite Interior Angle.

Plug in the numbers and you'll get -

m<1 = 60 + 123

m<1  = 123