Need to figure solution for varible

[OMac]

I've tried searching online and here in order to find the solution for L. I'm pretty much a newbie although I should really work on that. So how in the world do I get this to find L. 

I've attached my worksheet, I don't know what I'm doing wrong. Thanks!

[Werner_E]

You have to provide a guess value, like L:=1in

You sure mean a:=L/2-18 in and NOT a:=(L/2 - 18)in

When defining I its not necessary to add in^4.  Give it a try and type  I= afterwards and you will see that in your sheet I ends up with unit in^8

Why do you define a and x the same? Obviously x-a=0 for each value of L that way

You must formulate a and x as functions of L, like a(L):=L/2-18 in

You have a unit mismatch. y should be length, but the term at the right hand side ends up with dimension time^2