python 3.x - How to calucate the number of steps in interative exponent -

def exp2(a,b):     if b == 1:         return     else: return a*exp2(a,b-1)

i want calucate number of program. accodring textbook, attached how calucate steps.

t(b) = 3 + t(b-1) = 3 + 3 + t(b-2) = 3k + t(b-k)

if it's done b-k=1 k=b-1

t(b) = 3(b-1) + 2 = 3b -1

i can't understand why first number 3. how can number 3 in beginning?

step 1: b == 1

step 2: a*exp2

step 3: b-1

the first round 3, can t(b) = 3 + t(b-1).

RT