A wave is traveling down a string at 21 m/s. (a) What is the wave's amplitude? (b) What is the wave's wavelength? (c) What is the wave's frequency? (d) What is the wave's period?

From the given information, the only thing we can tell is the wave's speed ... 21 m/s.

That also happens to be the product of it's (frequency) x (wavelength), but we can't say what either one alone is.

None of the items listed can be calculated without more information.