What is ##(9times 10^ { – 6} ) – ( 5times 10^ { – 6} )##?

##4xx10^-6##
The question “What is ##9x – 5x## ” would pose no problem at all!
We can only add or subtract like terms, which is exactly what we have, so we can write:
##9x-5x = 4x##
In this case we have ##9color(red)(xx10^-6) – 5 color(red)(xx10^-6)##
Although they are numbers, we can regard them as being like terms in exactly the same way as we did with the algebra.
##9## somethings – ##5## somethings gives ##4## somethings, as long as the ‘somethings’ are the same.
##9color(red)(xx10^-6) – 5 color(red)(xx10^-6) = 4 xxcolor(red)(10^-6)##