5x + 1=2x+19
5x + 1 -1 = 2x +19 -1
5x = 2x + 18
5x - 2x = 2x -2x +18
3x = 18
3x/3 = 18/3
x = 6
8(x+3)=x+15
8*x + 8*3 = x+15
8x + 24 = x+15
8x - x +24 = x -x +15
7x + 24 = 15
7x + 24 -24 = 15 - 24
7x = -9
x = -9/7
Le dernier est a verifier parce que ça me parait bizare
