Question 356817
{{{3x-9=70}}}
{{{3x=79}}}
{{{highlight(x=79/3)}}}
.
.
.
That's the real answer. 
It's unfortunate that some teachers nitpick over minor details and require answers in a certain format. 
If the answer is not {{{x=79/3}}} then it's an approximation. 
Sometimes approximations are good enough. 
Lines over repeating numbers are only used in algebra books and not in the real world. 
Unfortunately its nonsense like this that stifles the creativity, wonder, and excitement that should go along with learning mathematics.
Hopefully, this experience won't dull your son's enthusiasm for algebra and learning in general.