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Is that number in Coulombs?

Since the net charge is positive, there are more protons than electrons. Figure out how many protons 5.12 x 10^-17 C comes to, considering that one proton has a charge of 1.602 x 10^-19 C.

That's the excess. You know there are that many more protons than electrons. For instance, suppose you find out this number is 100 (it isn't). Then the number of electrons is x and the number of protons is x + 100. Since you are told the total number is 1540, you know 1540 = x + x + 100.

Solve that with your real numbers. That will tell you how many protons and electrons there are.

The mass is (# protons) x (mass of proton) + (# electrons) x (mass of electron)

sounds like homework difficulty... E=form of electrons P=form of protons E+P=1423 P-E=-3.68E18 2 equations, 2 unknown... i will permit u do uncomplicated algebra. pay interest to (-) signs and indications in 2d equation.