What is ##(9\times 10^ { - 6} ) - ( 5\times 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)##