Answer:
x = 0
Explanation:
collinear means A , B , C lie on the same straight line
given B is between A and C , then
AB + BC = AC ( substitute values )
11 + 2x + 10 = x + 21 ( collect like terms on left side )
2x + 21 = x + 21 ( subtract x from both sides )
2x - x + 21 = x - x + 21 ( simplify )
x + 21 = 21 ( subtract 21 from both sides )
x + 21 - 21 = 21 - 21 , that is
x = 0