All of the interior angles in a parallelogram add up to 360 degrees. The opposite angles of a parallelogram are equal to each other, so that means in this figure m∠C also equals 70°.
Add m∠A and m∠C.
70 + 70 = 140
Now subtract this value from 360 to find how many more degrees are "left" in this parallelogram.
360 - 140 = 220
Since m∠B and m∠D have to be equal to each other, you can divide 220 by 2 to find the measures of angles B and D.
220/2 = 110
m∠B = 110°
You could also have solved this problem an easier way because angles A and B are supplementary angles, meaning they add up to 180 degrees (they form a straight line).
By using the equation m∠A + m∠B = 180, you could have substituted 70 for m∠A and subtracted 70 from both sides to have the same final answer of m∠B = 110°.