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What I am doing wrong. I am getting error while using the keyword
You can't solve for a function but only for a variable. But of course you can assign the result to a function and define it that way.
The modifier "explicit" has no effect if all used variables are undefined anyway. I used "factor" to get a result more similar to what you expect.
But the possibilities to influence the representation and form of the result of a symbolic evaluation are very limited.