Every "take out the common factor" question is the same three moves: find the highest common factor of the terms, write it out the front, then write what's left of each term inside a bracket.
The Three Moves
One — find the highest common factor (HCF). Look at the numbers and the pronumerals. What's the biggest number that divides every term? Do all the terms share an x? Whatever they all share, that's your HCF.
Two — write the HCF out the front of a bracket.
Three — fill the bracket with what's left. Divide each term by the HCF and pop the result inside. HCF(leftover + leftover).
A Worked One
Factorise 6x + 9. The numbers 6 and 9 both divide by 3; there's no x in the second term, so the HCF is just 3. Take it out: 6x ÷ 3 = 2x and 9 ÷ 3 = 3, so 6x + 9 = 3(2x + 3). Expand it back — 3 × 2x = 6x, 3 × 3 = 9 — and you're home.
When the Common Factor Includes a Letter
Sometimes the terms share a pronumeral too. In 6x² + 9x, both terms have a 3 and at least one x, so the HCF is 3x. Take it out: 6x² ÷ 3x = 2x and 9x ÷ 3x = 3, giving 3x(2x + 3). Same three moves — you just grabbed a letter as well as a number.