Bonjour,

1)

12

12+10 = 22

22*12 =264

264-12² = 264 -144 = 120

120+15 = 135

2)

x

x+10

x(x+10 ) = x² +10x

x²+ 10x - x²  = 10x

10x + 15

10x+15 = 291

10x = 291 -15

10x =276

x = 276/10

x= 27,6