I am surprised that question 3a is a Sec 1 question when it should have been a Sec 2 question. It’s very difficult understanding it in Sec 1.

4d2 - 81
= (2d)^2 - 9^2
= (2d + 9) (2d - 9)

where we used the special formula

a^2 - b^2
= (a + b) (a - b)

which would only have been covered in Sec 2.

Alternatively we could do a cross table factorisation, but this is only covered in Sec 2 as well.

For the same reason, Q3b,

16x3 + 8x2 + 1

is also not supposed to be taught in Sec 1.

What is a cross table factorization?

It’s a method of factorising an expression in the form ax2 + bx + c where a, b and c are numbers, such as 2x2 + 5x + 2. Of course I can’t show the technique here because it needs to be written down in a diagram form.

You will learn this in Sec 2 only.