I have a general query about velocity of waves. We have a formula velocity of waves V = fλ ; f= frequency,λ= wavelength But in string waves it's V = √(F/μ) ; where F is tension force μ=linear mass density. I am confused with other formulas also(like sound wave velocity). Is the V = fλ the general one  , i.e we can use it in any case either sound wave or string. And then what is the need of the second formula above.

I have a general query about velocity of waves. We have a formula velocity of waves: $$ V = f\lambda,\quad f= \mathrm{frequency}, \ λ= \mathrm{wavelength} $$ But in string waves it's $$ V = \sqrt{\frac{F}{\mu}}, \quad F =\text{tension force}, \ μ=\text{linear mass density} $$ I am confused with other formulas also  (like sound wave velocity). Is the $V = f\lambda$ the general one, i.e can we use it in all cases, including sound waves and string vibrations?. If so, what is the need of the second formula above?