Explain how to use REPL

[sxhya]

When a problem is formulated about gcd or matrices, it would be nice to explain the reader how he could check their answer. The students whom I assigned solving these problems presented the solutions to me without checking whether they work at all. In the hint for the exercise, describe how REPL is opened (right click on ard document > Open In > REPL), and what should be typed in the console.