Let's call x to the number of tulips you can buy
Each tulip costs $0.75, then x tulips cost 0.75x
There is also a $3.00 shipping fee. Then, the total cost of x tulips is 0.75x + 3.
Your budget is $15, then:
0.75x + 3 = 15
Solving for x:
0.75x = 15 - 3
0.75x = 12
x = 12/0.75
x = 16