Collinear means that these points form a line segment...or that B is on segment AC. So AB + BC = AC.
So x2+x2+ 5 = 3x2+7x+17
0 = x2 + 7x + 12
0 = (x+3)(x+4)
so x=-3 or x=-4.
If x=-3, your segment become 9, 14 and 23.
If x=-4, your segment becomes 16, 21 and 37.
So you have 2 possible answers.
Hope this helped.