D equals 38.2 I think because t equals 9.55. You would add -16t with -4t and get -20t. Which would turn into D=-20t^2+382. Since you can't multiply 2 with -20t, you would divide it by 382 which would be 191. D=-20t+191 with this, you would subtract 191 from +191 which would cancel out and it would turn the problem into -191=-20t. You would divide -20 from itself and -191 and get t=9.55. Then you plug t where it was in the problem and then you get that D=38.2. I hope my answer is right and it helps you.