Program for Derivative of a Polynomial

Given a polynomial as a string and a value. Evaluate polynomial's derivative for the given value. 
Note: The input format is such that there is a white space between a term and the '+' symbol

The derivative of p(x) = ax^n is p'(x) = a*n*x^(n-1)
Also, if p(x) = p1(x) + p2(x) 
Here p1 and p2 are polynomials too 
p'(x) = p1'(x) + p2'(x) 

Input : 3x^3 + 4x^2 + 6x^1 + 89x^0
 2 
Output :58 
Explanation : Derivative of given
polynomial is : 9x^2 + 8x^1 + 6
Now put x = 2
9*4 + 8*2 + 6 = 36 + 16 + 6 = 58 
 
Input : 1x^3
 3
Output : 27

We split the input string into tokens and for each term calculate the derivative separately for each term and add them to get the result. 

59

Time Complexity: O(n), where n is the number of terms in the polynomial.
Auxiliary Space: O(1)