From Highest and Lowest

A buoy rides waves between 1 m and 7 m above the sea floor. The midline sits halfway, and the amplitude is half the gap:

$$k = \frac{7+1}{2} = 4$$

$$a = \frac{7-1}{2} = 3$$

If the wave is on its midline at 0° and rising, it is $y = 3\sin\left(x\right)+4$: the sine starts on its midline going up.