Ok, shoot, and I will see what I can do for you. (I am certified to tutor math for Kindergarten- 4th grade.) But I do enjoy higher grade math challenges!
Just divide one by the other. Why did you ask your question in the "Answers" section instead of in the "Questions" section?
(7.5 x 10^-10) / (9.1 x 10-31) =8.24 x 10^20 electrons in 1 mote of dust.
"The mass of a dust particle is approximately 7.5x10^-10 kilograms
and the mass of an electron is 9.1x10^-31 kilograms.
Approximately how many electrons have the same mass as one dust particle?"
This is just a division problem. You want electrons are in each dust particle
So how many times 9.1x10^-31 kilograms. goes into 7.5x10^-10 kilograms
7.5x10^-10 kilograms divided by 9.1x10^-31 kilograms
(7.5x10^-10) / (9.1x10^-31)
=(7.5/9.1) * (10^-10 / 10^-31) electrons per particle
(7.5/9.1) = 0.8241758241758242 = 8.241758241758242*10^-1
(10^-10 / 10^-31) = 10^21
So tha answer is
\(\approx 8.24\times 10^{-1}\times 10^{21}\\ \approx 8.24\times 10^{20}\;\;\text {electons in one dust particle}\)