Please help me solve it
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10a- 4a= 6a        1) 6a+12
6xa= 6a   6x2=12    2) 6a+12

The expressions are equivalent. For the first sum, all you do is work out the total amount of a and what number is being added. However, on the second question, you have to expand the answer. I was taught a 'noddy method' to begin with. This should help.