It's a little unclear from the question what you mean, but I assume you mean the 'additive inverse'. In terms of solving equations, this term typically means we just minus 6x. i.e - 6x.
However, if the question means find the inverse function of f(x) = 6x,
then this would be a different answer: in particular, the inverse function is the function that 'undoes' this, so it would be f-1(x) = x/6.
