You must be aware that two adjacent sides are simply two lines that meet and form an angle in between them.
You are given that the sum of interior angles in a polygon (S) = 180°(n - 2) where n is the number of sides.
Therefore;
180°(n - 2) = 1260° (We are to find the value of n)
n - 2 = 1260°/180° = 7
n = 7 + 2 = 9
The polygon have 9 sides.