Answer:
"I write good music with an exclamation point!" declared composer Richard Wagner.
Step-by-step explanation:
There are two mistakes in the original sentence:
"I write good music with an exclamation point"! Declared composer Richard Wagner.
The first mistake concerns punctuation. The exclamation point belongs to the sentence inside the quotations marks. Thus, it too should be placed within those marks.
The second mistake concerns the capitalization of "Declared". There is no reason to capitalize it, since it comes after the direct speech.
Corrected, the sentence will look like this:
"I write good music with an exclamation point!" declared composer Richard Wagner.