Bonjour

f (x) = ( x - 2 ) ( x + 6 )

f (x) = x² + 6 x - 2 x - 12 = x² + 4 x - 12

( x + 2 )² - 16 = ( x + 2 - 4 ) ( x + 2 + 4 ) = ( x - 2 ) ( x + 6 )

f (x) ≤ 0

( x - 2 )( x + 6 ) ≤ 0

x ≤  2  ou  x ≤ - 6

f (x ) ≤ 20

x² + 4 x - 12  ≤ 20

x² + 4 x ≤ 20 + 12

x² + 4 x ≤  32

x ( x + 4 ) ≤ 32

x ≤ 2 ou x + 4  ≤ 32 d'où  x ≤ 28

f (x)  ≤ x² + 4

x² + 4 x - 12 ≤ x² + 4

x² - x² + 4 x ≤ 4 - 12

4 x ≤ - 8

x ≤ - 2