Express the following quantities as an ordinary decimal number:

Question

a) The sun has a diameter of 1.392 * 10^6
I get 1392000.
b) 8.4*10^-6
I get 0.000084
Is this right

The first one is correct. The second one is incorrect. Your answer is 8.4 *10^-5

you have 8.4*10^-6; start where the decimal is and count six times to the left because you have a negative exponent. This means the number is smaller than 1. So to left of the decimal we have 8 and since we have
10^-6, we add five zeros to the left of 8. Now you can see that :
0.0000084 is six digits to the left of 4. This number is less than 1.

The sign of the exponent is really what's important. I wrote 10^-6, it is of course 8.4*10^-6 or 8.4E-6 (more convenient)

I hope this helps...

The diameter of the sun you have written correctly. The number of 8.4 x 10^-6 needs one more zero than you have; i.e., you have 4 zeros and you need five.