This is an odd problem. The length and width look like they should be interchanged. Anyway I'll solve it and we'll talk about the results.
(x +7) (x + 3) = 396
x^2 + 7x + 3x + 21 = 396
x^2 + 10x + 21 = 396 Subtract 21 from both sides.
x^2 + 10x = 396 - 21
x^2 + 10x = 375
x^2 + 10x - 375 = 0 This probably factors.
(x + 25)(x - 15)
x + 25 = 0
x = - 25 which makes no sense. Negatives do not describe room dimensions.
x - 15= 0
x = 15. this is fine.
x+3 = 15 + 3 = 18
x + 7 = 15 + 7= 22
Check
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Area = L * W = 18 * 22 = 496. It does check.
22 should be the length
18 should be the width.