iteration.js

function multy(arr) {
    let result = 0;
    for (var j = 0; j < arr.length; j++) {
        result += arr[j][0] * arr[j][1];
    }
    return result;
}
const arr = [[5, 6], [2, 3], [1, 2]];
console.log(multy(arr));

//or more cool

function multy(arr) {
    let result = 0;
    arr.map(arr => {
        result += arr[0] * arr[1]
    });
    return result;
}
const arr = [[5, 6], [2, 3], [1, 2]];
console.log(multy(arr));