To begin, remember that the sum of all angles must equal 180.
With that being said, take your two known angles into account.
1 - X = Unknown
2 - 102
3 - 48
To find x, simply add 102 and 48.
102 + 48 = 150, the sum of angle 2 & 3.
Remember that the sum of the whole figure must be 180. With that being said, subtract 180 and 150.
180 - 150 = 30
x = 30
To double check, add all the sides together.
102+48+30 = 180