Why does what Meg sees make her angry? Why does Meg thank Mrs. Whatsit's, Mrs. Who's, and Mrs. Which's friend for making her angry?

The Happy
Medium wants to cheer up the children after showing them the Dark Thing and Camazotz. However,
her plan backfires. She reveals Calvin's unhappy home life in her crystal, and then she shows
Meg her mother. Instead of making her happy, the vision makes Meg angry, because her mother is
huddled up sadly in her lab at home. She has dropped her head to the paper she was writing on,
and her unhappiness is apparent. This is a side of herself she never lets her children
see.

Meg, who has been protective of those she loves throughout the book,
again wants to jump into action, this time to help her mother. Meg's response to her mother's
sadness is the following:

"Let's go!" she cried
harshly. "Let's do something!"

Meg's anger gets
her into trouble, but it also motivates her to take positive action to help people, which is why
Mrs. Whatsit tells her to hang on to it.