If #color(white)("XXX")S=n/s(A+t)#
then #color(white)("XXX")S * s/n = cancel(s)/cancel(n) * cancel(n)/cancel(s) (A+t)#
and #color(white)("XXX")S * s/n - t =A#
Note: if #S# and #s# are suppose to be the same variable then: #color(white)("XXX")A=(s^2)/n-t#