Для a) y = 7:
7 = -3x + 7 => -3x = 0 => x = 0
Для б) y = 52:
52 = -3x + 7 => -3x = 45 => x = -15
Для в) y = -8:
-8 = -3x + 7 => -3x = -15 => x = 5