i need help doing scientific notation?

Question

(7.8×10^4)(5×10^-6)

 

I mean i get how to do some of the problem. i know i have to multiply 7.8 and 5 and i get 39 but idk what to do with exponents. could someone help please. do i subtract 4 and -6? 

Answer 1

1) Multiply the numbers: (7.8 x 5) = 39

2) Since the numbers in step one were multiplied, then, add the exponents: (4) + (-6) = -2

3) Answer: 39 x 10^-2 ⇒ 3.9 x 10^-1

 

** If the numbers in step 1 were divided (7.8 / 5) then you would subtract the exponents, (4) - (-6) = 10.