help me

the points will be collinear if the area of the triangle formed with these points as vertices is zero.

So you can use the formula for area and get the answer.

Also,

 if you take these points as A,B,C respectively,

then slope of AB must be equal to slope of ACwhich also must be equal to slope of BC.

SO

6-2/-5-1=-2-6/a+5

2/-3=-8/a+5

2a+10=24

2a=14

a=7