to solve for percent by mass of a solution, we have the following formula: 

Pm = (mslt / msln) x 100 

Pm is percent by mass 
mslt is mass of solute 
msln is mass of solution (that means, mass of solute + mass of solvent) 
in words, the equation is: 
percent by mass is equal to mass of solute multiplied by mass of solution multiplied by 100 

mass of sodium chloride (NaCl) = mass of solute = 3.785 grams 
volume of water (volume because it's in terms of mL = 43.7 mL 
density of water = 1.0 g/mL 

we need all given to be in terms of grams because the formula requires "mass" not volume 
so, the volume of water must be converted to its mass 

to compute for the mass of water let us derive the formula from the formula of density 

d = m/v 
v = m/d 
v = 43.7/ 1 
so volume is 43.7 grams (it is given that when the substance is water volume is equal to mass) :-) 

mslt = 3.785 
mslv (solvent) = 43.7 
therefore, msln (solute + solvent) = 3.785 + 43.7 = 47.485 

Pm = (3.785/ 47.485) *100 
= 0.079709381910076866378856480993998 * 100 
= 7.9709381910076866378856480993998 
= 7.97% 

okay? thanks

Percentage by mass of solutes divided by the total mass of the solution times 100.
% by mass of solute= mass of solute  x 100
                              mass of solution