You need to calculate how many 2 person combinations can be made from 6. The formula is n! / [r! * (n-r)!] 6! / [2! * 4!] = 6 * 5 * 4! / 2! = 15 The answer to the second part of the question is also 15. Source: http://www.1728.org/combinat.htm
Multiply top so y values cancel out, so multiple top by -3. Y values cancel out. You combine top and bottom equations, x=2. Plug 2 in first equation for x. Y=4