You can estimate both numbers to the nearest tens but then the value would go off by more than a thousand so it is much more sensible to only estimate 503.
503-->500
7-->7
We'll keep 7 that way because, as I've said above, it would change the value too much if we rounded it to 10.
500 X 7 = 3500
That is close to the value of 503 X 7 which is equal to 3521.
You could also just round 3521 to 3520 but that would be rounding the result, not the expression itself.