Answer:
b) AE = 7
d) m∠DCB = 120°
f) m∠ADC = 60°
Explanation:
Part b
The diagonals of a parallelogram always bisect each other.
Therefore, point E (the point of intersection of the two diagonals) is the midpoint of diagonal AC. So AE = EC.
As EC = 7, then AE = 7.

Part d
As the measure of the opposite angles of a parallelogram are equal, then m∠DCB is equal to m∠DAB. From inspection of the given parallelogram, we can see that m∠DAB = 120°. Therefore:
m∠DCB = 120°

Part f
Adjacent angles of a parallelogram are supplementary (sum to 180°).
Angle DAB and angle ADC are adjacent angles, so their sum is 180°.
Therefore:
⇒ m∠ADC + m∠DAB = 180°
⇒ m∠ADC + 120° = 180°
⇒ m∠ADC + 120° - 120° = 180° - 120°
⇒ m∠ADC = 60°