bonsoir

3 x + 12 = x² - 16

( 3 x + 12 ) - ( x ² - 16 ) = 0

3 ( x + 4 ) - ( x - 4 ) ( x + 4 ) = 0

( x + 4 )  [3 - ( x - 4 ) = 0

( x + 4 ) (  3 - x + 4 ) = 0

( x + 4 ) ( - x + 7 ) = 0

x = - 4 ou  7