Calculate the angle gamma in the triangle 'c'.

Step 1: Calculate the angle $\gamma$ in the triangle 'c'.
In the right-angled triangle 'c', the side adjacent to angle $\gamma$ is 6, and the hypotenuse is 15. We use the cosine function.

$$\cos(\gamma) = \frac{\text{adjacent}}{\text{hypotenuse}}$$ $$\cos(\gamma) = \frac{6}{15}$$ $$\cos(\gamma) = 0.4$$

To find $\gamma$, we take the inverse cosine:

$$\gamma = \arccos(0.4)$$ $$\gamma \approx 66.4218^\circ$$

Rounding to the nearest 0.1 degree:

$$\gamma \approx \boxed{66.4^\circ}$$