How do you expand brackets?

Let's have a look at a pair of brackets: (a+b)(c+d)To expand these brackets each value in one bracket must be multiplied by all values in the other bracket separately then added together. So we have: ac+ad+bc+bd *Using this formula we can look at more specific examples: (x+3)(2x+1)Now let a=x b=3 c=2x d=1 then substituting into * we have 2x²+x+6x+3 Which we must simplify to 2x²+7x+3