Number of Bit Operation in fast multiplication Algorithm

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Determine a Value for the constant $C$ and use it to estimate the number of bit operations needed to multiply two 64-bit integers using the Fast Multiplication Algorithm

Attempt/Analysis-:

Let me write the recursive equation for Fast Multiplication (Karatsuba_algorithm)

$f \left(2n\right)$=$3*f \left(n\right)+C*n$

As The bit operations involves 3 operation -:

$1 .\text{Shift operation} $

$2 .\text{Add operation} $

$3 .\text{Subtract operation} $

$\Rightarrow \text{I know that shifting $n$ places would require n bit operation.} $

$\Rightarrow \text{I know that Addition of 2$n$ bit number would require atmost $3n$ bit operation.} $

$\Rightarrow \text{I know that Subtraction of 2$n$ bit number would require atmost $3n$ bit operation.} $

I am unable to move forward ....please help me out...answer is $50$n