x²-2x-15=0 (x+3)(x-5)=0 Therefore: x=-3 and x=5

Look, that 2x = 3x - 5x. Substitute it for "2x" : [latex]x^2-2x-15=0 \\ \underbrace{x^2 +3x}_{x(x+3)} \underbrace{- 5x-15}_{-5(x+3)}=0 \\ \\ x(x+3)-5(x+3)=0 \\ \hbox{And now factor out} \ \ \ (x+3): \\ (x+3)(x-5)=0 \\ \hbox{And you've got two roots:} \\ x_1=-3 \\ x_2=5[/latex]