"A farmer has 1200 acres of land": c+ w+ s= 1200
"Because of market demand he will grow twice as many acres of wheat as of corn.": w= 2c
"It costs $45 per acre to grow corn, $60 for wheat, and $50 for soybeans": so c acres of corn, w acres of wheat, and s acres of soybean will cost 45c+ 50w+ 50s dollars.
Also "He has allocated $63750 for the cost of growing his crops." so we must have
45c+ 50w+ 50s= 63750.
Solve the three equations
c+ w+ s= 1200
45c+ 50w+ 50s= 63750
Since w= 2c, you can replace w by 2c in both other equations. You can then solve the first equation for s and replace s by that in the third equation. Solve that equation for s, then work back for the values of c and w.