Well, we know that BA and DC are lines connecting points and therefor they are straight, that means opposite angles are equal.

m/3 + 75 = m/2 + 64  
We don't like those fractions we're gonna get rid of them, Lowest Common Denominator is 6  
6(m/3 + 75) = 6(m/2 + 64)  
6(m/3) + 6(75) = 6(m/2) + 6(64)  
2m + 450 = 3m + 384  
Now separate the variables to one side and numbers to the other  
2m - 2m + 450 = 3m - 2m + 384  
450 = m + 384  
450 - 384 = m + 384 - 384  
66 = m  
Let's test that out by plugging it back into the original equation  
66/3 + 75 = 66/2 + 64  
22 + 75 = 33 + 64  
97 = 97  
Seems legit  
m = 66  

But why m? I think that's m as in "y=mx+b" or the slope of the line? Kind of a weird notation but honestly just ignore it. It's just a regular variable for now.