3,900 acres of corn and 3,300 acres of soybeans
Use c = corn acres and s = soybean acres. Then the hours are split by the equation:
0.5c + s = 5,250
and the fertilizer is split by the equation:
7c + 4s = 40,500
If you solve the hours for "s" and substitute into the second, you have:
7c + 4*(5,250 - 0.5c) = 40,500....clear the parentheses
7c + 21,000 - 2c = 40,500....subtract 21,000 from each side; simplify the left side
5c = 19,500....divide each side by 5
c = 3,900
and substitute that back into the first equation:
0.5 * 3,900 + s = 5,250....simplify the left side
1,950 + s = 5,250....subtract 1,950 from each side
s = 3,300