Comment by keko for Instead of "assigning" a value to $\partial g/\partial x \vert_{x=0}$ beforehand, it's easier to make the substitution afterward:sage: der_f.subs(x==0).subs(diff(g,x).subs(x==0) == 10) 12 Or in steps, mimicking the order you proposed:sage: what_i_know = diff(g,x).subs(x==0) == 10 sage: der_f.subs(x==0).subs(what_i_know) 12

That definitely did the work! Nevertheless, is there a way I can set the value of ∂g/∂x|x=0 beforehand, so that it automatically does the substitution ∂g/∂x|x=0 = 10 every time it finds that expression?