Anyone knows the answer?

Given the function f(x) = 3x + 1, evaluate f(a + 1). a. 3a + 1 b. a + 2 c. 3a + 4

With this one. will mark brainliest

Steve wants to develop a way to find the total cost of any item including sales tax. the sales tax in his city is 6%. his friend jenna writes the expression x + 6x where x is the cost of the item. explain whether her expression is correct based on the situation.