a level maths hypothesis testing q

Seedlings come in trays of 36. On average, 12 seedlings survive to be planted on. A gardener
decides to use a new fertiliser on the seedlings which she believes will improve the number
that survive.

State the probability of incorrectly rejecting H0 using this critical region

I know how to find the critical region but don't quite get the question.

You should know the null hypothesis probability and from a quick google its a 90%/10% significance level, though your question may be slightly differet It doesnt explicitly state it, but I guess theyre presuming the test is on a single tray.

What are you unsure about? Can you post a pic of the full quesition.

my bad, accidentally left out the significance level. I was confused about the final bit, 'State the probability of incorrectly rejecting H0 using this critical region'. Are they simply asking for the actual significance level? I guess I'm just unsure about the wording of the question and perhaps the meaning of actual significance level.

what did you get for b?

X is greater than or equal to 17

Agree with that. However, thats not a 10% significance level as its discrete, so there is a bit less than 10% chance of incorrectly rejecting the null hypothesis using that critical region.

10% level means there is 10% chance of incorrectly rejecting the null hypothesis - a so called type 1 error.