Question

density of 3M solution of Nacl is 1.25 g/L.The molality of this solution is?

Answer 1

Molar mass NaCl = 58.44g/mol 
3mol = 3*58.44 = 175.32g 
Take 1.0L or 1000mL of the solution 
This has mass = 1250g ( from density) 
Mass of water = 1250-175.32 = 1,074.68g H2O 

You have 3 mol NaCl dissolved in 1.07468kg water 
Molality = 3.0/1.07468 = 2.79m 

Answer 2

m = 1000*M / (1000*d - M*molecular weight)
given,
 M=3
d=1.25
molecular weight = 58.5

m =  1000*3/(1000*1.25 - 3*58.5)
m =   3000/1250-175.5
m =   3000/1074.5
m =   2.79199