Let p = her pay per week.
"Karissa earns $200 per week plus $25 per item she sells."
She earns a fixed amount of $200 per week no matter how much she sells.
Then she earns an additional variable amount that depends on the number of items she sells. It is $25 per item.
p = fixed amount + variable amount
The variable amount is:
If she sells 1 item, she earns 25 * 1
If she sells 2 items, she earns 25 * 2
If she sells 3 items, she earns 25 * 3
If she sells n items, she earns 25 * n
The variable amount she earns is 25n, where n is the number of items she sells.
p = fixed amount + variable amount
p = 200 + 25n