One complete turn of the double helix of DNA contains 10 pairs of bases and is 3.4 nm long What is the approximate length of the DNA coding sequence of lysozyme a protein of 129 amino acids?

Well, we would have to know how many % of the coding sequence would be exons and introns, but let's ignore this and assume 100% of the sequence are exons;

129 aminoacids = 129 codons = 129*3 nucleotides = 387 nucleotides.

10 pairs of bases = 3,4 nm;

387/10 = 38,7

38,7 * 3,4 = 131,58 nm.