Trenton earnings are $190/week + $1.50/item sold.
Let x represent the number of items he sold and y represent his total earnings then:
y=190+1.50x
This means that in a bad week, if he sells no items (x=0) he will earn $190, and each time he sells one item (x increases one unit) he earns + $1.50
If his goal is to earn $550, you hace to replace y=550 in the formula and solve it for x:

To earn $550 Trenton needs to sell 240 items