Question: Substitute an isolated variable into an expression

Hi

im working with a complicated system of equations and am tryin to find steady states, ive set the equations equal to zero and isolated a particular variable, i now want to sub this variable back into another equation, is there a way i can use the isolated variable without copying the answer and pasting it into 'subs'? (the experssion for the variable is very long and messy!)

Thanks!

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