First four terms are: -12, -24, -48, -96
============================================
Step-by-step explanation:
The notation f(1) = -12 tells us that the first term is -12.
The equation f(n) = 2*f(n-1) indicates that we're multiplying the (n-1)th term by 2 to get the nth term. Basically we multiply each term by 2 to get the next term.
-12*2 = -24
-24*2 = -48
-48*2 = -96
So the first four terms are -12, -24, -48, -96.
-----------------------------------
Alternative Approach
f(1) = -12 .... first term
f(n) = 2*f(n-1) ... nth term recursive formula
f(2) = 2*f(2-1) ... plug in n = 2
f(2) = 2*f(1)
f(2) = 2*(-12) ... plug in f(1) = -12
f(2) = -24 ... second term
f(n) = 2*f(n-1) .... resetting for the next computation
f(3) = 2*f(3-1) ... plug in n = 3
f(3) = 2*f(2)
f(3) = 2*(-24) ... plug in f(2) = -24
f(3) = -48 .... third term
f(n) = 2*f(n-1) ... one more reset
f(4) = 2*f(4-1) .... plug in n = 4
f(4) = 2*f(3)
f(4) = 2*(-48) ... plug in f(3) = -48
f(4) = -96 ... fourth term
We get the first four terms to be -12, -24, -48, -96, which was the same as earlier.