To factorise, we ask: what are the factors of the x^2 term? 3 and 1; and the factors of the constant term? 5 and 1. 3*5+1*1=16, the coefficient of the middle term, so this tells us where to put the numbers into the factor brackets. 3 and 5 must be in different factor brackets so that leaves the 1's, one in each bracket. We also have to get 16 with an x term, so that tells us that we have to have 3*5*x+1*1*x to give us 16x, the middle term. To get 3x^2 we must 3x times x, and to get 5 we must have 1 and 5 without x. We end up with (3x+1)(x+5).