Supergirl 10 (Sept/Oct 74) came out after Supergirl's series had already moved to Superman Family, so it could be seen as more of a Supergirl special, released in the summer. Bates, Saaf and Colletta provide both of the stories in this issue.
The cover story brings back Prez, whose comic had been cancelled a few months earlier. In this story he is still the President of the US, and is visiting a nearby town. Supergirl is watching on tv, and spots an assassin. She zips over to save Prez. The story shows his devotion to reparing watches, but that's about the only element of his own series that plays in this tale.
Supergirl protects Prez from another assassination attempt, but this is all playing right into the villain's hands. The goal was to get Supergirl near Prez, and then use voodoo to take control of her, and make her kill him. This appears to work, as seen on the cover, and it looks like Supergirl drops Prez to his death.
In fact, she dropped a dummy, then used her vision powers to spot the voodoo-cannon thing that was beaming into her brain, and take out the bad guys. The final panels of the story are the last time Prez is seen for twenty years, until Neil Gaiman brought him back in Sandman.
The final Supergirl story in her first solo book creates a cloned brother for her.
A scientist tricks Supergirl into entering his lab, stealing cells from her with the intention of creating a clone he can control. His machines were not set for Kryptonians, and the clone winds up coming out male.
The scientist sends Super Kid, as he calls him, to go out and steal for him. Supergirl quickly gets involved. The boy is able to match Supergirl in powers and strength, but does not understand why she is angry with him.
They both wind up returning to the scientist. Super-Kid learns that he was created, and is not a real person. The scientist tries to get the boy to kill Supergirl. Conflicted, he uses the weapon on himself, to remove himself from existence. It's a real downer note to go out on.
But Supergirl's new series keeps running in Superman Family.