One half the sum of four consecutive multiples of 6 is 6 more than twice the 3rd highest of the multiples. What is the sum of the two highest multiples?

I solved this by hand first, then cymath, then Wolfram alpha. we all agree the solution to the equation involved is x=-3 however - 3 is not a multiple of 6. if you plug it in you get a solution of 24 but I don't consider this to be a valid question.