Your answer would be 21 because being that the two angles must add up to be 180 you can subtract 180-48 which gives you 132 then divide that by 6 which gives you 22 finally subtract 1 which equals 21. In conclusion of all this X=21
You can check this by inputing 21 into the problem so:
6(21+1)=132
132+48= 180
