NOTE: I'll assume you mean sodium chloride, #"NaCl"#, instead of #"NaC"#.
We're asked to find the percent by mass of #"NaCl"# in a solution.
We can use the equation
#%"mass" = "mass solute"/"mass solution" xx 100%#
We're given the mass of the solute is #4.35# #"g"#, so we need to find the mass of the solution, which is
#"mass solution" = overbrace(4.35color(white)(l)"g")^"mass of NaCl" + overbrace(105color(white)(l)"g")^"mass of water" = color(red)(109# #color(red)("g"# (obeying significant figure rules)
We therefore have
#%"mass NaCl" = (4.35color(white)(l)"g NaCl")/(color(red)(109color(white)(l)"g soln")) xx 100% = color(blue)(3.99%#
