-21+3z/2 divide 121z-847/4

You need to use parentheses. Otherwise the sequence of operations is not clear, and many different interpretations and results are possible.

Do you mean
[(-21+z)/2]/(121z-847)/4] ?

That would be 2(z-21)/(121z-847)
= (2/121)(z-21)/(z-7)