Consider that to take into account the number of flowers Tina is going to have, it is necessary to use the subtraction.
Tina picks 18 flowers, after she throws away 3 flowers, she has:
18 - 3 = 15
now, she has 15 flowers.
Next, she gave to her mother 8 flowers, after this, she has:
15 - 8 = 7
Hence, Tina has now 7 flowers: