Wrong: "If 6 Was 9." (Axis, The Jimi Hendrix Experience)
Right: "If 6 Were 9."
Most commonly, were is simply the past tense of the verb was. But you may need to check if the verb is used in the subjunctive mood to ensure you don't replicate Jimi's mistake above.
What's the subjunctive?
A verb (in this case, was) is in the subjunctive if it expresses an action or state that is not real — that is, if it's hypothetical, wished for or conditional.
In the above example, 6 and 9 are numerals with specific values; one can never be, nor stand in for, the other. Consider the following:
I wish I was as good a guitarist as Jimi Hendrix.
That's never going to happen. As such it's a hypothetical statement, and should read I wish I were as good a guitarist as Jimi Hendrix.