



 
Hi Guido, You need to treat $L,$ the length of the beam as a constant rather than a variable. What you will end up with is an expression that gives the value of $x$ that yields the maximum deflection for any beam length $L.$ For the differentiation I want to look at a similar expression $f(x) = k x^3  7 x^2 + k^3 x  4k.$ Treating $k$ as a constant and $x$ as the variable I get \[f^{\prime}(x) = 3k x^2  14 x + k^3\] Try the problem now and write back if you need more assistance,  


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