how do i solve 4/r5+6/rt divided by 12/st
Your notation is unclear. It seems like you have three unknowns: r, t and s, and no equation. (An equation has an = sign.) It cannot be solved.
An exponent is indicated by "^" online. For example, do you mean 4/r^5 or (4/r)^5?
Repost in clearer terms.