Find the value of a so that the points (1,4),(2,7),(3,a) are collinear.

Explanation

As the point (1,4),(2,7),(3,a) are collinear we have the area of the triangle with vertices (1,4),(2,7),(3,a) is zero.
`therefore` `1/2{1(7-a)+2(a-4)+3(4-7)}=0`
or, 7-a+2a-8+12-21=0.
`implies` a=10.