Write a piecewise function that models the cellular phone billing plan described below. Let x represent the number of minutes used and C(x) represent the cost for those x minutes. Then graph the function. $30.00 per month buys 350 minutes. Additional time costs $0.20 per minute. Fill in the missing values or expressions for the piecewise function below. if O sx S C(x) (Simplify your answer. Do not include the $ symbol in your answer.)