a) 5.(–8).2.(–3)
= 5.2.(-8).(-3)
= 10.24
= 240
b) 3.(–5)² + 2.(–5) – 20
= 3.(–5)² + 2.(–5) – 4.5
= (-5).[ 3.(-5) +2 +4]
= (-5).[(-15) +2 +4]
= (-5).(-9)
= 45
c) 34.(15 –10) – 15.(34 –10)
= 34.15 -34.10 -15.34 +15.10
= 15.(34 -34) -10.(34 -15)
= 15.0 -10.19
= 0 -190
= -190