Answer:
He factored out a 2 from both (20x+18) and (20x+18), which is the same as factoring out a 4 from the whole expressions. It should be 4(10x+9)(10x-9) .
Step-by-step explanation:
Firstly, he should find the common factor between 400 and 324, so 4 is the common factor between.
400x² - 324 = 4(100x² - 81)
Next, both 100 and 81 is a perfect square so he can factorised using the formula, (a+b)(a-b) = a² - b² :
4(100x²- 81) = 4(10x+9)(10x-9)
