Step 1. We need to find two integers m & n that their product is mn=ac=9 where a=9 and b=6 and their sum m+n=6.
Step 2. After trial an error the integers are m=n=3. Then based on the method of grouping where 6x=3x+3x based on the integers m & n, we have
Step 3. Factoring 3x out of the first group we have
Step 4. Now factor (3x+1) from both groups since it's common to both of them.
ANSWER
I hope the above steps were helpful.
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