Step-by-step explanation: To start things off, let's find the m<2.
Notice that <4 and <2 are vertical angles since they share
a common endpoint and their sides form opposite rays.
Vertical angles are always congruent so we know that the m<4 = m<2.
Since the m<4 = 51, the m<2 must also be 51.
Now, notice that <1 and <2 form a straight angle
which always adds to 180 degrees.
So we have the equation 51 + x = 180.
Solving from here, we find that x = 129.
So the m<1 = 129.
Since <1 and <3 are again vertical, we know that m<3 = 129.