Factor just means look for something in common between the terms of an expression. In this case 3 goes into 9 and 12, so 3 is a factor. Write 3( because we are going to divide each term by 3 and write the result inside the bracket: 3(3x+4). This makes use of the distributive property: what’s outside the brackets is applied to each term in the brackets. So the answer is 3(3x+4).