First four terms in this sequence plz help?

I got frustrated.

a1= 2; a2=-1;
an = an-1 + an-2

I understand that you have to plug in a1 and a2 into that equation, but What is throwing me off is an the extra a2 that they are giving to me.

Can someone plz explain how to do this?

Thanks!

You are given the first two terms.

a3 = a(3-1) + a(3-2)
a3 = a2 + a1 = -1 + 2 = 1
a4 = a(4-1) + a(4-2) = a3 + a2 = 1 + -1 = 0

an = an-1 + an-2 MEANS the value of a term a(n) is equal to
the sum of the two preceding terms, a(n-1) and a(n-2).

Hence

1, 1, 2 ...... because 2 = 1 + 1

then

1, 1, 2, 3, 5, 8, ...

Hope this helps you!