∠∠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!