Bonjour,
1.) 75 + 4x =150 + x
4x - x = 150 - 75
3x = 75
x = 75/3
x = 25
2.) 5 ( x - 1 )+ 10 = -3 ( 3x -4 ) + 8
5x - 5 + 10 = - 9x + 12 + 8
5x + 9x = 12 + 8 + 5 - 10
14x = 15
x = 15/14
3.) 2x² + 25x - 19 = 2x² - 15x + 21
2x² - 2x² + 25x + 15x = 21 + 19
40x = 40
x = 40/40
x = 1.