Aliana has $42.50
if she wants to buy a sweater that cost $29 
and some pairs of socks $4.50 each
in this case 
x is the number of pairs of socks that Aliana can buy 
so the equation is
29+4.50x=42.50
 we need to clear x 
4.50x=42.50-29
4.50x=13.50
x=13.50/4.50
x=3
she can buy 3 pairs of socks