Explanation:

- The boat's speed is combined with the stream's speed when traveling downstream.

- Conversely, the stream's speed is subtracted from the boat's speed when moving upstream.

- Let the boat's speed in still water be \( x \) km/h.

- Downstream speed = \( x + 3 \) km/h and time taken = 5 hours.

- Upstream speed = \( x - 3 \) km/h and time taken = 7 hours.

- Distance calculation for both:

$$ \text{Distance} = \text{Speed} \times \text{Time} $$

- For downstream: \( (x + 3) \times 5 = \text{Distance} \)

- For upstream: \( (x - 3) \times 7 = \text{Distance} \)

- Setting equations equal:

$$ 5(x + 3) = 7(x - 3) $$

- Solving for \( x \):

$$ 5x + 15 = 7x - 21 $$

$$ 36 = 2x $$

$$ x = 18 $$

- Thus, the speed of the boat is 18 km/h