am you me i’m stuck on these questions

Carson has only $20 bills and $10 bill in her wallet. the total value of the bills is $50. she has 1 more $20 bill than $10 bills. how many each kind of bill does carson have? a. one $20 and two $10 bills b. two $20 bills and one $10 bill c. one $20 bills and three $10 bills d. two $20 bills and two $10 bills

For f(x)=2x+3 and g(x)=x^2- 8 find (f+g) (x)

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?