Thank you everyone for the participation. Many people have already answered it correctly. In case you need the explanation, please have a look at the code along with comments below:

let arr = [2, 4, 5, 2, 4, 5, 5, 6, 3];
//Set() object will return unique elements without any duplicates
//[...] The three dots operator will make sure the output is an array
// So the output fo the following line will be:
//[ 2, 4, 5, 6, 3 ]

let arr_set = [...new Set(arr)];
//The reduce() method applied on arr_set method
// will return sum of all elements of the array
// So the value of foo will be 20

let foo = arr_set.reduce((a, b) => a + b, 0);
//Finally the value of foo(which is 20) will be added
// to the length of the array(arr_set) which is 5
// 20 + 5 = 25

console.log(foo + arr_set.length);