Sadie simplified the expression StartRoot 54 a Superscript 7 b cubed EndRoot, where a greater-than-or-equal-to 0, as shown colon StartRoot 54 a Superscript 7 baseline b cubed EndRoot = StartRoot 3 squared times 6 times a squared times a Superscript 5 Baseline times b squared times b EndRoot = 3 a b StartRoot 6 a Superscript 5 Baseline b EndRoot Describe the error Sadie made, and explain how to find the correct answer.