(x+3) (x-1)
Take the first term from the first bracket that is x and times it by the second bracket. x(x-1)
now take the second term from the first bracket that is +3 and times it by the second bracket. +3(x-1)
so you have
$$\\ x(x-1) +3(x-1)\\
=x^2-1x+3x-3\\
=x^2+2x-3$$