The term 2.1 × 10^2 has 2 significant figures, so the final answer should be rounded to 2 significant figures.

Substituting x = 2 into the equation gives 5(2) + 3y = 15, which simplifies to 10 + 3y = 15. Thus, 3y = 5, and y = 5/3.

Substituting x = 2 into the equation gives 5(2) + 3y = 20. This simplifies to 10 + 3y = 20, leading to 3y = 10, thus y = 10/3, which is approximately 3.33. The closest integer option is 4.

2.5 has 2 significant figures and 3.456 has 4 significant figures. The result should be rounded to 2 significant figures, giving 8.6.

First calculate inside the parentheses: 3 + 2 = 5. Then, 5 × 2 = 10. Finally, 10 - 10 = 0.

Following the order of operations: 10 - 6 + 4 = 8.

Subtracting 4 from both sides gives 2x = 12. Dividing by 2 gives x = 6.

Adding 7 to both sides gives 3x = 9. Dividing by 3 gives x = 3.

Expanding both sides gives 4x - 8 = 3x + 3. Subtract 3x from both sides: x - 8 = 3. Adding 8 gives x = 11.

The number 3.0 has 2 significant figures, so the result should also be reported with 2 significant figures.

Rounding 0.006789 to three significant figures gives 0.00679.