Z-test and T-test

Which one do I use!?!

Stuck in the middle of a tedious assignment,
In despair, my tortuous soul cry out and lament!
My brains cells all run dry
What juice left have I
To crawl to freedom, from all these torment!

Overcame many a conundrum I can attest,
None as troublesome upon me as this one behest,
Utterly confused, confounded!
Speechless and dumbfounded!
Who the devils invented the Z and T-test!

Bravely I took on my darkest trepidation
I rush forward with all my indignation!
With fury like never before
A pen from my bag case I tore
And began on the nastiest multiplication!

Attempting to solve this blasted mathematic
I end up ranting, foaming, oh how pathetic
Falling like others on this path
To overcome his demonic math
Laying in defeat, I have become but another statistic