TESTCASE SEEMS TO BE WRONG!

I tried using airthmatic series in simple way but testcases keep failing..please clear me if I am wrong.

question :

Given 3 numbers A,B,C find the sum of Arithmetic Series with a=A, d=B and n=C
Sample Testcase :
INPUT
1 1 2
OUTPUT
3

code:

let a = userInput[0].split(' ').map(a=>+a);

let b = ((a[2]-1)*a[1])+ a[0];

console.log(b);

error :

Testcase Status:

>>> error

Input:

3 3 7

Expected Output:

84

Actual Output:

21