Let’s resolve into factors

(a + b)²-5a-5b + 6

(a + b)²-5(a + b) + 6 … (1)

To resolve it into factors we first resolve 6

6=2×3

Given, x² + (p + q)x + pq=(x + p)(x + q) … (2)

Comparing 1 and 2

p + q = -5

pq =6

∴ (1) can be written as

(a + b)² – 3(a + b) - 2(a + b) + 6

= (a+b)(a+b-3) -2(a+b-3)

=[{(a + b)-3}{(a + b) -2}]

∴ the resolved factors are [{(a + b)-6}{(a + b) + 1}]