a5+b5 could also be written like this: 5a+5b because expressions are usually written with the number first and letter second.
If you take a look at the expression you will notice there is a variable and a coefficient:
5 : coefficient
a : variable
5 : coefficient
b : variable
The variable is the letter. It is called the variable because whatever the value of the letter is, the value for the whole expression changes as well.
The coefficient is the number before the variable. The coefficient signifies the amount of the variable there are. So here's an example:
5a = a+a+a+a+a
5b = b+b+b+b+b
The operation in between the two figures is the addition sign (+).
This means the question is asking us to add the two figures.
But, the variable of the two figures are different - so the value of those numbers aren't the same. This means we cannot add the two figures together.
Or in other words, the question is asking us to do this:
a+a+a+a+a+b+b+b+b+b = ?
However, as the variables are different, we cannot collect like terms so the most we can do is leave the expression like this:
5a+5b
(Hope the answer helped!)
