Second degree equations always have two (not necessarily different) solutions. We are told that 9 is one solution to w² - 81 - 0. To find the other, we can solve it for w and look for both.
w² - 81 = 0
w² = 81
w = √81
w = 9 OR w = -9
When solving by square roots, you need the solution and its opposite. That's because while 9 * 9 = 81, (-9) * (-9) = 81 as well.
The other solution is -9.