What is the total negative charge, in coulombs, of all the electrons in a small 0.900 g sphere of carbon? one mole of c is 12.0 g , and each atom contains 6 protons and 6 electrons?

The first thing you have to do is find the number of mols. That's done in terms of the masses.
mol = mass of sample / molar mass
mol = 0.900 / 12.0
mol = 0.0750 mol Carbon

Next Each mol of carbon contains 6 mol of electrons. 
mol electrons = 6 * mol of carbon 
mol electrons = 6* 0.0750 = 0.450 mol

Next find out how many electrons are in 0.450 mol
1 mol = 6.02 * 10^23 
0.450 mol  = 6.02 * 10^23 * 0.45 = 2.71 * 10^23 electrons

Finally find the charge in coulombs
1 electron has a charge of 1.60 * 10 ^- 19
2.71 * 10^23 electrons have a charge of 1.6*10^-19 * 2.71*10^23
The answer is 43360 Coulombs = 4.34 * 10^4 coulombs. 

I have deliberately not used textbook formulas because each textbook has its own format.