Question 833
OK, we start by putting together an equation that shows what we know:

{{{3(9 + 2x)=4x + 13}}}

Then we can work through and solve it:

{{{27 +6x=4x+13}}}
{{{2x=-14}}}
{{{x=-7}}}

So you were correct - the answer is -7. Putting this back into the original equation gives:

{{{3(9+2*(-7))=4*(-7) + 13}}}
{{{3*(-5)=-28 + 13}}}
{{{-15=-15}}}

It works. So your error must have been somewhere in the calculations when you substituted that value back in. Check over your work again, to see where you went wrong. This is one of the reasons you should always show your working. :-)