The product of the constants in each bracket must be 8. 4*4=16 so we know the answer can't be B. The signs in the brackets have to be different because we have -8, so one must be plus and the other minus. The answer can't be A or C. Therefore the answer is D, but we still have to factorise the expression. We need 3x and x in separate brackets to make 3x^2. The factors of 8 are 1 and 8, and 2 and 4. These are the possibilities to get 2 for the coefficient of the middle term (2x): (3x 1)(x 8), (3x 8)(x 1), (3x 2)(x 4), (3x 4)(x 2), but the signs have been deliberately left out for the moment. These give the following coefficients: 3*8~1=23, 3~8=5, 3*4~2=10, 3*2~4=2. The twiddles sign (~) just means the difference (either positive or negative). The sign in front of the 8, which is minus, tells us to find the difference. If it had been plus, we would have found the sum instead. Only one set of factors, (3x 4)(x 2), gives us the required 2. All that remains is to find out where the plus and minus go in the brackets. We have -2x and the minus tells us that minus must go in front of the number giving us the larger product, 3*2=6 or 4*1=4, so the former is larger and the minus goes in front of 2, so the answer is (3x+4)(x-2).