Hey there!
The degree of this binomial would be 1.
To find the degree of a binomial, you must find the term in the expression that is raised to the highest power.
To do this, first look at both terms you are given with in the expression:
a and 8
The term, a, would be raised to the first power as any variable to the 1st power would be itself. The term, 8, would be raised to the 0 power as any constant has a degree of 0. The reason for this is because imagine 8 has an x variable attached to it:
8x
The x must be raised to the 0 power in order for 8x to remain 8 (anything raised to the 0 power is 1):
8x^0=8(1)
=8
Therefore, because the term x has the highest degree, 1, your degree of the binomial would be the 1st degree.
Hope this helps! :)