Why do I have to take the moment at the cut to get the right answer?

[Svelte1]

Homework Statement

Need to set the derivative of m(x) to 0.

Relevant Equations

taking moments..

Every time I have taken moments before I have been able to solve for the required unknown regardless of what point I choose, unless there is more than 1 unknown! However I tried taking moments from the far left point load of 97.5 this time and I get the wrong answer. I don't understand why? Thankyou! This image details the correct answer taking the moment from the hypothetical cut so as to find the internal moment values.

 

[Orodruin]

If you take the moment around any other point than x then also the force at the cut will influence the result because it will get a non-zero lever. You likely neglected this but it is impossible to know unless you show your work.

 

[Lnewqban]

There is zero internal moment at the left end.
Please, see:
https://mathalino.com/reviewer/mech...tion-to-problem-410-shear-and-moment-diagrams