The equation is .

               (Subtract 42r from each side)

The perfect square of the given trinomial .

                    (Take the square root of each side)

Apply square roots property: If then .

Apply addition property of equality: If a = b then .

                            (Add 7 to each side)

                (Apply additive inverse property: )

                             (Divide each side by 3)

The solution is .