Do you mean ter tell me,“ he growled at the Du…

Do you mean ter tell me,“ he growled at the Dursleys, “that this boy—this boy!—knows nothin’ abou’—about ANYTHING?”
Harry thought this was going a bit far. He had been to school, after all, and his marks weren’t bad.
“I know some things,” he said. “I can, you know, do math and stuff.