21x = 196 – x2

(viii) 21x = 196 – x²

Sol. 21x = 196 – x²

∴ x² + 21x – 196 = 0

∴ x² – 28x – 7x – 196 = 0

∴ x (x + 28) – 7 (x + 28) = 0

∴ (x + 28) (x – 7) = 0

∴ x + 28 = 0 or x – 7 = 0

∴ x = –28 or x = 7