Answer:
<4 = 81
Step-by-step explanation:
we know that all the interior angles of a triangle will equal 180
so <1 + <2 + <3 = 180
we know that <4 is the exterior angle to <3
the exterior angle is equal to the angles opposite to its interior
<3 is the interior angle to <4
<1 and <2 are the angles opposite to <3
so <4 = <1 + <2
<4 = 42 + 39
<4 = 81
