Hi!
Let x = the number of sunflowers
y = the number of petunias
Since each one of the sunflowers makes one flower, and each of the petunias makes 5, and your mom wants 55, then I would write the equation as:
x + 5y = 55
The amount of sunflowers (since they all equal one there's no need to multiply x with anything) plus the amount of petunias • 5 (since each petunia makes 5 flowers) = 55 total flowers.
Hope this helped!