Trees help keep air fresh by absorbing carbon dioxide. The city of Normal has $2100 to spend on planting spruce and maple trees. The land available for planting is 45,000 square feet. Spruce trees cost $30 to plant and require 600 square feet of space. Maple trees cost $40 to plant and require 900 square feet of space. Spruce trees absorb 650 lb/yr of carbon dioxide and maple trees absorb 300 lb/yr of carbon dioxide. Write a linear programming formulation to decide how many of each tree should be planted to maximize carbon dioxide absorption?