PLEASE HELP ME!!

consider an interaction between two mutually inhibiting proteins with concentrations x and y, given by the differential equations:

dx/dt = f(y)-x

and

dy/dt = g(x)-y

where both f(y) and g(x) are decreasing functions.

*show that equilibria occur when f[g(x)]=x

how do i do the asterick part? any help would be very much appreciated..