1 is a curve: Y=C+I+G, C = 100+0.9 (1-t) Y, I = 200-500r: y =100+0.72y+

Y= 12500/7- 12500r/7

2.LM curve: from l = m/p: 0.8y+200-2000r =1000r: y =100+2500r.

3. At equilibrium, IS=LM, that is,12500/7-12500r/7 =100+2500r, that is, R = 59/15, y.

4. Bring the income Y at equilibrium into C= 100+0.9*( 1-t)Y to calculate consumption, and bring the interest rate at equilibrium into i= 100+2500r to calculate investment. There is no net export problem in this three-sector economy, so the net export is 0. I hope I can help you.