add Euler problem 6 (#470)

Project-Euler/Problem6.js

@@ -0,0 +1,15 @@
+// https://projecteuler.net/problem=6
+
+const num = 100 // number we are checking; change to 10 to check 10 from example
+
+const squareDifference = (num) => {
+  let sumOfSquares = 0
+  let sums = 0
+  for (let i = 1; i <= num; i++) {
+    sumOfSquares += i ** 2 // add squares to the sum of squares
+    sums += i // add number to sum to square later
+  }
+  return (sums ** 2) - sumOfSquares // difference of square of the total sum and sum of squares
+}
+
+console.log(squareDifference(num))