An isolated conductor has a net charge of +12.0 × 10- 6 c and a cavity with a particle of charge q = +3.70 × 10-6
c. what is the charge (a) on the cavity wall and (b) on the outer surface?

This is just a simple problem finding out the outer surface charge, the inner surface charge and the net charge. Net charge by definition means the difference between two charges. In this case, the formula that is applicable here is outer surface charge = total net charge - inner cavity surface charge. Since we are given already with the net charge equal to 12.0 x10-6 C and the inner charge magnituude f 3.7 x10-6 C, the the total charge must be outer charge is +10x10(-6)) - (-3.0x10(-6)) = +1.3x10(-5) C. 
Charges are measured in coloumbs and most likely exist on surfaces of entities like particles, walls etc.

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