Write the likelihood function for the expected number of mutations per million bases in the first piece

Question:

Write the likelihood function for the expected number of mutations per million bases in the first piece and find the maximum likelihood estimator.
Mutations are counted in four pieces of DNA that are 1 million base pairs long. There are 14 mutations in the first piece, 17 in the second piece, 8 in the third piece, and 5 in the fourth piece.