X=81
Firstly, you’ve to know 55, 40 and x are numbers on a straight line. And number on a straight line sum up to 180 degrees . Add 55 and 40 equal to 99. Since X is an alphabet. It can’t be added to the numbers 99.
Note: 99+x=180degrees .
You group like terms
X= 180- 99
X=81
