D is the answer............
The binomial expansion:

a = 2y, b = 4 x^3, n = 4
( x )^3k = x^ 9
k = 3


Answer: the coefficient is
512.
I believe the equation is y=350+150x with y being the total amount of money in his account and x being the number of months..Paul starts with $350, which is the y-intercept (starting value) of the equation then the slope is 150 because his total savings increases by $150 for every month he saves without making any withdrawals. I apologize if I'm wrong but I hope this helps.
Answer:
3
Step-by-step explanation: