Posted by Jake on Thursday, March 29, 2012 at 8:23pm.
what's the problem?
(k,2k-1,3)•(k,5,-4) = k*k + (2k-1)(5) + 3(-4)
so, we need
k^2 + 10k - 5 - 12 = 7
k^2 + 10k - 24 = 0
(k-2)(k+12) = 0
so, k=2 or -12
k=2: (2,3,3)•(2,5,-4) = 4+15-12 = 7
k=-12: (-12,-25,3)•(-12,5,-4) = 144-125-12 = 7