Question: how can i sort a expression by index

how can i sort this expresion by index of L

f(15) := expand(((673*L[13] - 485*L[14] - 176*L[15])*h^2 - 90*(L[13] - (13*L[14])/15 - (2*L[14])/15)*x*h)/(12*h^3));

thank you

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