Squaring a Binomial

Question

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: $$ 2ab = (a^2 + b^2) - (a - b)^2 $$

Answer 1

$$ a^2 - b^2 = (a+b)(a-b) = 21 \Rightarrow (a+b)\times 3 = 21 $$

Can you finish the calculation from here?

Answer 2

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