Squaring a Binomial How to find ab if:
$$
a - b = 3
$$
$$
a^2 - b^2 = 21
$$
What I already know is the following:
To find ab for:
$$
a - b = 3
$$
$$
a^2 + b^2 = 29
$$
ab is:
$$
2ab = (a^2 + b^2) - (a - b)^2
$$

From $a-b=3$ and $(a-b)(a+b)=21$ we get $a-b=3$ and $a+b=7$ and $a=5$, $b=2$. 
