A gardener crosses a true-breeding tall pea plant with a true-breeding dwarf plant…

Step 1. Tall plants. Three parts out of four are tall: $\tfrac{3}{4} \times 1200 = 900$ plants.

Step 2. Dwarf plants. One part out of four: $\tfrac{1}{4} \times 1200 = 300$ plants.

Step 3. Tall plants that breed true. Only TT plants breed true. TT is one part out of four of the F2: $\tfrac{1}{4} \times 1200 = 300$ plants. The other $600$ tall plants are Tt, and they give dwarf plants as well.