On your TI-84 calculator:

Press STAT ENTER and make it read like this:

L1      |L2       |L3
-----------------------------
 1      | 2       |
 3      | 5       |
 4      | 9       |
 5      | 19      |

Then press STAT and the right arrow key to highlight CALC
Scroll down with the down-arrow key to 

0:ExpReg

Press ENTER

You'll see this:

     ExpReg
Xlist:L1
YList:L2
FreqList:
Store RegEQ:
Calculate

Use the down-arrow key to scroll down to highlight 'Calculate'

Press ENTER

You'll see

y=a*b^x
a=____________
b=____________
r²=___________
r=____________

I won't tell you what numbers will be in those blanks, so you'll
have to do it yourself instead of turning in my answer.
Round them off to four decimal places.

y = _._____*__._____^x

Edwin