What are the consequences of government spending by imposing tax?
Your professor want probably that you compute the keynesian multiplicator (hint: once you have the general formula, replace dT by dG and see what you get).

Intuition: if you raise G, you will raise your GDP. As Y increases, your consumption increases. But the taxes (T, lump sum) increase also, so the consumption decreases (it depends of the disposable income). The overall effect is a 1 to 1 increase of your GDP.