separate the sentence with a line between the complete subject and the complete predicate

Three-fourths of the earth / is covered by water, and most of it is salty ocean water.
is the line in this simple sentence correct??
THANKS

You've put the line in the right place in the first clause, but what about the second clause in that sentence?

This is not a simple sentence. It's a compound sentence with two subjects and two predicates.

http://www.eslbee.com/sentences.htm

To separate the sentence with a line between the complete subject and the complete predicate, you need to identify the complete subject and the complete predicate in the sentence.

The complete subject of the sentence is "Three-fourths of the earth," and the complete predicate is "is covered by water, and most of it is salty ocean water."

So, to separate the sentence, you would write it as:

Three-fourths of the earth
is covered by water, and most of it is salty ocean water.

Therefore, the line in this simple sentence is correct.