You can solve this very easily if you know what the Bernoulli law is.

You have two opposite results:

Let's call the probability to get head: p = 0.1

Let's call the probability to get tail: q = 0.9

Since 1-0.9=0.1 the results are opposite. (First condition to apply Bernoulli Law)

The number of times we toss the coin is called: n = 10

Each toss is independant from the previous one. (Second condition to apply Bernoulli Law)

Finally we call the number of times we get head: x

In that case: