Cant solve this

Question

One of you friends gives you $10 for a charity walkathon. Another friend gives you an amount per mile. After 5 miles, you raised $13.50 total. Write an equation that represents the amount y of the money you have raised after x miles

Answer 1

$13.50=$10.00+5a where a=amount per mile

$13.50-$10.00=5a

$3.50=5a

a=$3.50/5=$0.70

y=$10.00+$0.70x