Need help with my pattern PLEEEEEASE!

I am making my a jumper and on the front, currently working on the neck divide and have come to these instructions:
Dec 1 st at neck edge of 2nd and foll 9 alt rows and at same time
inc 1 st at side seam edge of 4th and foll 8th row.
17 [20: 24] sts.

Please can you explain how this works because i dont understand how i am meant to reduce the number of stitches i have when i am increasing and decreasing by two
Thank you

Welcome to the forum!
It looks like you’re decreasing on a total of 10 rows but only increasing on 2 rows.
Decrease at the neck edge on rows 2,4,6,8,10,12,14,16,18,and 20. Increase at the side seam edge on rows 4 and 8.
What are you making? Can you tell us a pattern name?

Thank you so much!
Its a Rowan pattern by Sarah Hatton called Courtney

Oooh, very nice!

It’s going to be lovely, I’m sure.

Hello again
sorry, am a fairly new knitter, i can do socks, gloves and hats fine but this is my first jumper. The next section have confused me slightly again:

Dec 1 st at armhole edge of next 3 [3: 5] rows, then on foll 1 [3: 3]
alt rows and at same time dec 1 st at neck edge of 3rd [next: 3rd]
and 0 [2: 2] foll 4th rows. 9 [9: 10] sts.
Dec 1 st at neck edge only on 2nd [4th: 4th] and every foll 4th row
until 4 [5: 6] sts rem.

So i have done the decrease of the next 3 rows but am unsure about the next section, does this mean knit 6 rows 3 of which have a decrease or 3 rows and one decrease. Then the next bit i don’t understand eitherabout the following 4th row.
Sorry to be a pain

Which size are you making, the 1st, 2nd or 3rd?

I am doing the second size

Decrease at the armhole edge on rows 1,2,3, and then rows 5,7,9. At the same time, dec at the neck edge on rows 1,5,9 and then rows 13,17,21, and 25.
These directions are often confusing. Just ask when you’re unsure. It’s no trouble at all.