The garden has space for 6 * 13 = <<6*13=78>>78 flowers in total.
Wilma has two times more green flowers than yellow flowers, which means she has 2 * 12 = <<2*12=24>>24 green flowers.
That leaves us with 78 - 24 - 12 = <<78-24-12=42>>42 red flowers in the garden.
#### 42