Question

r^2-22r+21=0

Answer 1

Factor the left side into two factors.
( )( ) = 0
Since r^2 = r*r, you put an r on the left side of each parentheses.
(r )(r ) = 0
Now you need two factors of 21 that add up to -22.
Can you think of two numbers that multiply to +21 and add up to -22?
You put them in the right side of the parentheses.