Question 899939
You have too many equal signs in the equation. 
If it's 
{{{7a+5a-6a=24}}}
Then,
{{{6a=24}}}
{{{a=4}}}
.
.
.
If it's 
{{{7a+5a=6a-24}}}
{{{12a=6a-24}}}
{{{6a=-24}}}
{{{a=-4}}}