Answer:
b = -7
and
b = -9
The dust particle has 400 times more mass than the pollen particle
Explanation:
To answer this problem we use scientific notation.
We know that a number like:
0, 002
It can be written as:

Where 3 is the number of times the comma (,) must be run to the right to convert the quantity 0, 002 to 2.
0, 002
0| 0 0 2
0 0| 02 -----------1
0 0 0|2------------2
0 0 0 2| ----------3

Therefore, for the given expression:
Powder particle 0.000000778 g
It can be written as:
0.0000008 g

b = -7
In the same way
Pollen grain 0.00000000155 g
It can be written as:
0.000000002 g

b = -9
To know how big is the mass of the dust particle with respect to the pollen divide:

The dust particle has 400 times more mass than the pollen particle