Which frequencies between 10 ghz and 20 ghz will create standing waves in the microwave cavity?

To create standing waves, the length of the cavity should be:
L = nλ/2
λ = 2L/n , where n is a natural number
You should insert the value of L into this equation and then substitute different values of n, starting from 1 and increasing by 1 everytime, to obtain frequencies that lie within the given range.