The sum of adjacent angles in a parallelogram equals 180. This is because the total of all 4 angles is 360 and opposite angles are equal, so any two adjacent angles sum to 180.
Let x = one of the angles and y = the other one.
x + y = 180
With the given relationship, then 7x = 3y and y = (7/3)x. Substitute this into the other equation.
x + (7/3)x = 180
(10/3)x = 180
x = 180(3)/10 = 54
y = (7/3)(54) = 126
Hope that helps!