(9 + √10)(9 - √10)

Use the foil method when multiplying two binomials.

(9 + √10)(9 - √10)

= 9(9) + 9(-√10) + √10(9) + √10(-√10) {used foil method}

= 81 - 9√10 + 9√10 - √100 {multiplied through}

= 81 - √100 {combined like terms}

= 81 - 10 {evaluated square root of 100 to be 10}

= 71

- Algebra House