Find the nonpermissible replacement for a in this expression 3/4a

Under the assumption your expression is , a cannot equal 0.  If a=0, then 4a=0 and we have division by 0, which is undefined.
If your original expression was , a could be any real number.

You can use a calculater and divide the minutes by the $$, hope that you

The variables should add up to equal 1