\[\sqrt{b - 1} - \sqrt{8 - b} = 2\]
\[\left( \sqrt{b - 1} - \sqrt{8 - b} \right)^{2} = 4\]
\[7 - 2\sqrt{(b - 1)(8 - b)} = 4\]
\[- 2\sqrt{(b - 1)(8 - b)} = - 3\]
\[\sqrt{(b - 1)(8 - b)} = 1,5.\]