Good evening Candice! Here are my workings for this question. There is, however, a slightly more advanced idea which can shorten my workings heavily based on what we call product of roots (from the topic sum and roots of a quadratic equation, which is out of the A Maths syllabus from this year's Sec 3 onwards).

This is because at the x-axis (for points A and B, with roots a and b),

x² - 6x + 8 = 0

Without factorising this, I can tell that the product OA x OB is basically the product of the two roots ab, which I can read off as constant term / coefficient of x² which equals 8.

Similarly, at the y-axis (for points C and D, with roots c and d),

y² - 9y + 8 = 0

Without factorising this, I can tell that the product OC x OD is basically the product of the two roots cd, which I can read off as constant term / coefficient of y² which also equals 8.

If you don't understand my typewritten workings above, it's ok, you can use the one I have written out using purple ink. But if you don't understand my purple workings as well, let me know and I will explain them again.