Seven distinct points are identified on the circumference of a circle. How many different triangles can be formed if each vertex must be one of these 7 points?
We can rephrase the question, and say: Out of 7 points, how many ways can we choose 3? The key word here is "choose". This tells us to use the choose formula.
What you should do is solve for 7 choose 3.
If you don't know how, refer to this: https://www.thoughtco.com/derive-the-formula-for-combinations-3126262