AMST 1100 Lecture 22: biopolitics and THX film analysis

A standing wave is formed in a stretched string that is 3.0 m long.What are the wavelengths of (a) the first harmonic and (b) thesecond harmonic?

my instructor mentioned using f1=V/2L for the first andL=n(wavelength/2) for the second but I am unsure how to find the Vfor the first one.

Ethanol fuel mixtures have "E" numbers that indicate the percentage of ethanol in the mixture by volume. For example, E10 is a mixture of 10% ethanol and 90% gasoline. How much E4 should be mixed with 3500 gal of E10 to make an E8 mixture?