To find how many triangles can be formed by joining 12 points in a plane, where 7 of these points are collinear (i.e., on the same straight line), we need to break the problem into two steps:

Step 1: Calculate the total number of triangles from 12 points

In general, to form a triangle, we must choose 3 points from the 12 given points. The formula to choose 3 points from $n$ points is the combination formula: $C(n, 3) = \frac{n!}{3!(n-3)!}$ For $n = 12$: [ C(