For the function f(x) = 4x - 1, what is f(21)?

For the function f(x) = 4x - 1, what is f(21)?

• A. 79
• B. 85
• C. 83
• D. 87

Answer: (C)

Evaluating a function at a given input means plug the number into the function’s rule and simplify. For f(x) = 4x - 1, replace x with 21. That gives 4 × 21 − 1. Multiply first: 4 × 21 = 84. Then subtract 1: 84 − 1 = 83. So the value is 83. This shows applying the rule of a linear function to the input step by step: multiply by 4, then subtract 1.

Evaluating a function at a given input means plug the number into the function’s rule and simplify. For f(x) = 4x - 1, replace x with 21. That gives 4 × 21 − 1. Multiply first: 4 × 21 = 84. Then subtract 1: 84 − 1 = 83. So the value is 83. This shows applying the rule of a linear function to the input step by step: multiply by 4, then subtract 1.