1) 48 = 11 - (9a + 2),
48 = 11 - 9а - 2.
48 = 9 - 9а,
9а = 9 - 48,
9а = -39,
а = -39/9 = -13/3 = -4 целых 1/3;
2) 13 - (5x + 11) = 6x,
13 - 5х - 11 = 6х,
2 - 5х = 6х,
-5х - 6х = -2,
-11х = -2.
х = 2/11;
3) 12 = (7x - 9) - (11 - x),
12 = 7х - 9 - 11 + х,
12 = 8х - 20,
-8х = -20 - 12,
-8х = -32,
х = 4;
4) 11x + 103 = 1 + (12x - 31),
11х + 103 = 1 + 12х - 31,
11х + 103 = 12х - 30,
11х - 12х = -30 - 103,
-х = -133,
х = 133;
5) видимо, в 1-й скобке пропущена переменная х.
(4 + 3) - (10x + 11) = 7 + (13 - 4x),
7 - 10х - 11 = 7 + 13 - 4х,
-10х - 4 = -4х + 20,
-10х + 4х = 20 + 4,
-6х = 24,
х = - 4.
0,75 - (x - 3/5) = 4,5
0,75 - х + 3/5 = 4,5
1,35 - х = 4,5
-х = 4,5 - 1,35
-х = 3,15
х = -3,15