№1
1) 7 ≤ a ≤ 10|·3; 21 ≤ 3a ≤ 30. 2) 7 ≤ a ≤ 10|-3; 4 ≤ a - 3 ≤ 7.
3) 7 ≤ a ≤ 10|·(-3); -30 ≤ -3a ≤ -21. 4) -30 ≤ -3a ≤ -21|+6; -24 ≤ 6 -3a ≤ -15.
4) 7 ≤ a ≤ 10|·2; 14 ≤ 2a ≤ 20|+3; 17 ≤ 2a+3 ≤ 23.
№2
P = 3a, 7 ≤ a ≤ 8|·3; 21 ≤ 3a ≤ 24; 21 ≤ P ≤ 24.