Supplementary angles are angles that add up to 180º.
If m∠1 is 70º, then m∠2 must be an angle that can be added to 70º to equal 180º.
In other words, m∠1 + m∠2 = 180º.
Since m∠1 is 70º, we can plug this value in the equation.
70º + m∠2 = 180º
To solve, just subtract 70 from both sides, which means 180 - 70 = m∠2.
180 - 70 = 110
m∠2 = 110
The measure of m∠2 is 110º.
Hope this helps!