Finding maximum average subarray of given length

I am trying to solve a Leetcode problem with JavaScript

#643 Maximum Average Subarray

My solution works for all the test cases but the time complexity is too high. How can I optimize this?

See Problem

var findMaxAverage = function(nums, k) {
    let maxsum = undefined;
    let sum = 0;
    for(let j=0; j<=nums.length-k;j++){
        for(let i = j; i<j+k; i++){
            sum+=nums[i]
        }
        if(maxsum <= sum || maxsum == undefined){
            maxsum = sum;
        }
        sum=0;
    }
    return maxsum/k;

Also I want to understand how to calculate the time complexity im guessing its O( (n-k+1) * k) please help me with this as well.

Thank you