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You can define your expressions as proper equations and use .subs()

var('rho V s a F m W')
eq_F = F == m*a 
eq_W = W == F*s
eq_m = m == rho*V
eq = W-rho*V*a*s

Thus

eq.subs(eq_W).subs(eq_F).subs(eq_m)

returns 0.