Difference of a complex number z=a+ bi and its conjugate is a real number

Which is the standard form for this number? (4 x 1 100 ) + (8 x 1 1,000 ) + ( 3 x 1 100,000 ) ? a) 0.004803 b) 0.04803 c) 0.0483 d) 0.483

Ineed to know the answer to this algebra question.

Write the expression 7x^3-28xy^2 as the product of 3 factors