Salut !!
A = (x + 5)²
A = (x + 5)(x +5)
A = x² + 5x + 5x +25
A = x² + 10x + 25
B = (x + 10)(x - 10)
B = x² + 10x - 10x + 100
B = x² + 100
ABC^ = x°
BAC^ = (x + 10)°
BCA^ = (x + 20)°
180° = x° + (x + 10)° + (x + 20)°
180° = 3x° + 30°
3x° = 180° - 30°
3x° = 150°
x° = 150/3 = 50°