Tim is now earning $500 per week after receiving an increase of $45 per week. what was tim's previous weekly salary

Now Tim gets $500 after it $45 was added to what he had last week

That means that
"What Tim earned last week" + $45 = $500

Subtract $45 from $500
$500-$45= $455

If you're not sure check your work
If I make $455 now and next week I get $45 more next week
$455+$45=$500
Next week I make $500