"Greatest Common Factor" requires more than one term. If "v2" (probably intended to be "v^2"), "-15v" and "56" are to be treated as three separate terms then please clearly specify this.

It may be that you simply wish to factor the expression "v^2-15v+56".

In either case, you should simply use our desktop software or the solvers at doyourmath.com or quickmath.com. Free trial versions of the desktop software are available through these links:

Windows: https://www.softmath.com/tr/alg51win-tr-en.exe

Mac: https://www.softmath.com/tr/alg51mac-tr-en.dmg