How many excess electrons must you add to an object to give it a charge of -6.50 μc?

Micro = 10^-6 
n = nano = 10^-9 

(a) -2.5 micro coulombs = -2.5 X 10^-6 
The charge on a electron is always -1.60 X 10^-19. 

So, given charge divided by charge on electron gives you number of electrons. 
(-2.5X10^-6)/(-1.60X10^-19) = 1.5625 X 10^13 excess electrons. 

(b) -2.5 nano coulombs = -2.5 X 10^-9 
The charge on a electron is always -1.60 X 10^-19. 

So, given charge divided by charge on electron gives you number of electrons. 
(-2.5X10^-9)/(-1.60X10^-19) = 1.5625 X 10^10 excess electrons. 

Round my answers to 3 significant figures if needed or to the appropriate form asked by the question.

The answer is true for your question.

The molecules contain like 54 of water