I believe it would be declarative because it is stating a fact.
Interrogative is a question and will allmost always end in a question marks.
Imperative is an order and will almost always end in a period.
Exclamatory shows excitement or concern ends most often with a exclamation mark.
If "y" represents the monthly salary of a salesman then we can rearrange the equation so that "y" is in evidence:
y=2000+100x
If he sells no computers and "x" is the number of computer systems he sells, then x=0.
y=2000+100*0 <=> y=2000+0 <=> y=2000
If the salesman sells no computers, his monthly salary is of 2000.
The answer is a direct object