One property of parallelograms is that the sum of the measures of each pair of adjacent angles equals 180°. Angles A and D are adjacent and so are angles B and C. So, the sum of the measures of the angle pairs must equal 180°. I will use the angle pair A and D for this problem.
m∠A = 70°
m∠D = x + 40°
m∠A + m∠D = 180°
(70) + (x + 40) = 180
x + 70 + 40 = 180
x + 110 = 180
x + 110 - 110 = 180 - 110
x = 70