All 3 angles inside the triangle should sum up to 180 degrees.
You already have 2 of the 3:
a) 48
b) 16x
To find the third one is simple. Since you know that m is a straight line, which means it is 180 degrees, you then take out 28x from 180.
So angle c) = 180- 28x
So now you have both a,b and c.
Since it is a triangle, a + b + c = 180
<=> 48 + 16x + (180 - 28x) = 180
<=> 228 - 12x = 180
<=> - 12x = 180-228
<=> - 12x = -48
<=> 12x = 48
<=> x = 48/12
x = 4