I just don't understand this question:

Question

h(t)=A*b^x h(3)=4 h(5)=40
Solve for b
Would you plug 3 into the x and set it equal to 4? (4=A*b^3) How would you solve for that because in the next question they go on to ask you to solve for A using the b you found...

Almost looks like your textbook has made a typo.

Yes, if this is a typo, you should "plug 3 into x and set it equal to 4".

You have two constants,a and b, and two equations to solve them.

H(3)=4=Ab^3
H(5)=40=Ab^5

divide the first equation into the second.

40/4=Ab^5/Ab^3

10=b^2
solve for b, then go back and substitute and solve for A.