You can find this button on the right or left side of your keyboard. You can find this button on the bottom right and left row of your keyboard, to the left and right of the Spacebar. The Number Pad will consist of a block of the numbers from 0-9 and you can typically find it to the right of your keyboard. You can also use 960. Once you're done typing the numbers and have released the Alt key, you will produce the symbol π. Press the key again to turn it off. This will return your keyboard to its previous settings.

TL;DR in Vietnamese:
Nhấn vào phím Num ⇩. Nhấn và giữ phím Alt. Sử dụng bàn phím số, gõ 227. Thả phím Alt ra. Tắt Num ⇩ bằng cách nhấn vào phím này một lần nữa.