Introducing himself at the start of last night’s Republican presidential debate in Nevada, Herman Cain said he was a “42-year businessman, which means I solve problems for a living.’’ One problem he’d better solve soon is how to defend his trademark “9-9-9’’ tax-reform plan from the criticism heaped on it by every other candidate in the GOP race. Cain’s proposal - which would replace the current federal tax code with three new taxes of 9 percent each on personal income, business profits, and sales - has drawn fire from the right practically from the day it was unveiled. In a critique on Monday, the editors of National Review labeled it “Bold, Brash, and Wrong,’’ and said its weaknesses “render it unworthy of conservative support.’’ Cain should have been ready with a compelling defense of “9-9-9’’ when he walked onto that Las Vegas stage yesterday. He wasn’t.