i've calculated N using formula above and then switch into the formula N=(m/M)x N(a), where N(a) is Avogadro's number , M is the mass number which is 131 and N has been calculated as perviously .the answer i get for m is 1.1x10^-14 Kg but the text book says the answer is 1.9 x 10*-13 Kg .
hope I'm in the right track .