+1094 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
:)
+1094 Noori, the answer is what guest said. But, guest rounded it, so that's probably why you got it wrong. In fraction form, it is 66/7. (like @below)
Also, you should thank Dragan for their diagram. It is not the same as the one in the picture you put up. Though you may not see it, Dragan has put the two similar triangles in his/her picture (and here is their explanation: https://web2.0calc.com/questions/help-please-thx_4.) Since the two lines are parallel, it means WXY is a right triangle. (please correct me if I'm wrong) From there, you should be able to constuct proportions, and find the length. (that is how I did it, but there are many ways to solve the same problem, and my way was pretty slow)
:)
+1094 Since this is a monic quadratic, we can use vieta's formula : https://artofproblemsolving.com/wiki/index.php/Vieta%27s_Formulas.
So, we see that mn=n+m, and -m-n=m. Now, here is where is get's a bit tricky. When we add m to both sides of the second equation, we see that -n=2m. Divide by -1, we see that n=-2m.
Here is what I want you do to: plug -2m for n in the first equation, then solve for m.
Then, put that into n=-2m, and solve for n.
Good luck!
:)
Credit: gwenspooner85