How do you use algebra to solve #(x-1)(x+2)=18#?
We know that a if a product is
Let's try to make it what we have:
Great!, Except -- now we don't have a product on the left. Try something and see if it helps or makes things worse.
(I have experience, I often know what will work. A less experienced person has to just try something.)
It is clear that there are 2 solutions: