$$F=mg\sin\theta=(508\text{ N})(9.8\text{ m/s}^2)\sin26^\circ=234\text{ N}$$

The acceleration of the crate is:

$$a=\frac{F}{m}=\frac{234\text{ N}}{508\text{ kg}}=0.461\text{ m/s}^2$$

The crate's initial velocity is zero, so its final velocity after 6 seconds is:

$$v=at=0+\left(4.29\frac{\text{ m}}{\text{ s}^2}\right)\left(6\text{ s}\right)=25.7\text{ m/s}$$

Therefore, the crate will be moving at 25.7 m/s after 6 seconds.