$45x +$50y =$2,500
Since you want to know how many men's shoes he must sell to make $2500 , plug 0 in for x pairs of women's shoes.
$45(0) + $50y =$2,500
0 + $50y =$2,500
$50y = $2,500
$50y/50= $2,500/$50
y = 50
He must sell 50 pairs of men's shoes to still earn his weekly income of $2,500
Check
$45(0) + $50(50) = $2,500
$0 + $2500= $2,500
$2500 = $2500