I need help finding the minumum number of residues.. Not sure what I'm doing wrong?

Under the proper environmental conditions, the salt-loving archaeon Halobacterium halobium synthesizes a membrane protein (Mr 26,000) known as bacteriorhodopsin, which is purple because it contains retinal. Molecules of this protein aggregate into "purple patches" in the cell membrane. Bacteriorhodopsin acts as a light-activated proton pump that provides energy for cell functions. X-ray analysis of this protein reveals that it consists of seven parallel -helical segments, each of which traverses the bacterial cell membrane (thickness 45A). Calculate the minimum number of amino acid residues necessary for one segment of an helix to traverse the membrane completely. minumum number of residues = residues