We have r rows and t trees per row. We're give two equations:

- rt = 378
- r = t + 3

(t + 3)t = 378

Multiply through:

t^2 + 3t = 378

We have a quadratic equation. To solve this equation, we type it in our search engine and we get:

t = 18 and t = -21

Since t cannot be negative, we get trees per row (t):

**t = 18**