split coefficient of , ie. 3, into its factors 3x and 1x. also split the constant -5 into -1 and +5
you will find that:
3 x -1 = -3
1 x +5 = +5
putting these two products together you have (-3) + (+5) = (+2), which is the coefficient of x. however when you fill up the brackets with these factors you don't write (3x - 1)(x + 5). You write (3x + 5)(x - 1).