Dưới đây là cách giải từng bài toán tìm \( x \):

### Bài 3: Tìm x
a) \( x + 3.12 = 14.6 - 8.5 \)
\[
x + 3.12 = 6.1 \implies x = 6.1 - 3.12 = 2.98
\]

b) \( x - 5.14 = (15.7 + 2.3) \cdot 2 \)
\[
x - 5.14 = 18 \implies x = 18 + 5.14 = 23.14
\]

c) \( 3 \cdot 31.5 - x = (18.6 - 12.3) \cdot 3 \)
\[
94.5 - x = 18.3 \implies x = 94.5 - 18.3 = 76.2
\]

d) \( x \cdot 12 - 5 = (32.6 - 10.4) \cdot 5 \)
\[
x \cdot 12 - 5 = 11 \cdot 5 \implies x \cdot 12 - 5 = 55 \implies x \cdot 12 = 60 \implies x = 5
\]

### Bài 4: Tìm x
a) \( 0.35x + 1.2 = 4.6 - 3.1 \)
\[
0.35x + 1.2 = 1.5 \implies 0.35x = 0.3 \implies x = \frac{0.3}{0.35} \approx 0.857
\]

b) \( [(-4.3) + 2x] + 0.4 = 2.7 \)
\[
-4.3 + 2x + 0.4 = 2.7 \implies 2x - 3.9 = 2.7 \implies 2x = 6.6 \implies x = 3.3
\]

c) \( 2.7x + 4.3x - 1.25 = 93.0 - 25 \)
\[
7x - 1.25 = 68 \implies 7x = 69.25 \implies x = \frac{69.25}{7} \approx 9.893
\]

d) \( (-12.5)x + 1.2 = -(2.1 - 0.5) \)
\[
-12.5x + 1.2 = -1.6 \implies -12.5x = -2.8 \implies x = \frac{2.8}{12.5} = 0.224
\]

### Bài 5: Tìm x
a) \( 0.73x + 3.5 + 0.3 = 3.3 + 0.023 \)
\[
0.73x + 3.8 = 3.323 \implies 0.73x = -0.477 \implies x = -0.653
\]

b) \( x \cdot 4.7 + 5.3 = 4.5 \cdot 0.3 - 0.7 \)
\[
4.7x + 5.3 = 1.35 - 0.7 \implies 4.7x + 5.3 = 0.65 \implies 4.7x = -4.65 \implies x \approx -0.989
\]

c) \( 13.7 + 18 + 3.2x = (67 - 0.4) \cdot 2 \)
\[
31.9 + 3.2x = 66.6 \implies 3.2x = 34.7 \implies x = \frac{34.7}{3.2} \approx 10.84375
\]

Bạn có thể kiểm tra các giá trị x nếu cần.