Answer : The length of protein will be, 94.1 A⁰
Explanation :
First we have to calculate the amino acid residue.

Given:
Length of single strand of protein = 69.0 kDa
Mean residue mass = 110 Da

Now we have to calculate the number of turns in protein.

As there are 3.6 amino acid per turn of the alpha-helix.

Now we have to calculate the length of the protein.

As, we know that the length of each turn = 0.54 A⁰

Thus, the length of protein will be, 94.1 A⁰