∠∠2+∠M+∠7 = 180,∠ 1+∠5+∠B= 180
∴∠M+∠7=∠5+∠B
∠∠3+∠6+∠M = 180,∠4+∠D+∠8= 180
∴∠6+∠M=∠8+∠D
∠5=∠6, ∠7=∠8.
∴2∠M=∠B+∠D
That is ∠M= 1/2(∠B+∠D).
The corner has been marked in the picture.
I wish you success in your studies!