What is the missing part of the solution?

6.64

Explanation:
In the solution, we have a 10^-19.
This means that we will need to take a 10^-19 as a common factor from all terms.
For the first term:
3.4 * 10^-20 = 0.34 * 10^-19
For the second term:
5.4 * 10^-19 already has a 10^-19 in it
For the third term:
9 * 10^-20 = 0.9 * 10^-19

Therefore, the given expression:
(3.4*10^-20) + (5.4*10^-19) + (9*10^-20)
can be rewritten as:
(0.34 * 10^-19) + (5.4 * 10^-19) + (0.9 * 10^-19)
Take 10^-19 as a common factor and compute the sum as follows:
(0.34 * 10^-19) + (5.4 * 10^-19) + (0.9 * 10^-19)
= 10^-19 (0.34+5.4+0.9)
= 6.64 * 10^-19

Hope this helps :)

= 10^-19 (0.34 + 5.4 + 0.9)
= 6.64 x 10^-19