49a²+14a18b+81b²
When you square a binomial, you have to multiply the entire thing by itself, so in this case it should be: (7a+9b) (7a+9b).
Now you have to use FOIL (or First, Outside, Inside, Last) to simplify the equation. When you set up FOIL, it should look like this:
7a×7a, 7a×9b, 7a×9b, 9b×9b. Simplify this and you should get 49a²+7a9b+7a9b+81b². Add like terms and you get 49a²+14a18b+81b².