Question

a patrat -b patrat=45
a+b=15
5a-5b=?

Answer 1

$a^2-b^2=(a+b)(a-b)=45 \\ a+b=15 \\ 15(a-b)=45 \\ a-b=3 \\ 5a-5b=5(a-b)=5*3=15$

Answer 2

a² - b² =45  
(a+b)(a-b) = 45
a+b = 15
inlocuim in ecuatia initiala : 
15(a - b) =45
a-b = 45 : 15
a-b = 3

5a -5b = 5(a - b) = (inlocuim pe "a-b" cu 3) 
5 × 3 = 15 => 5a - 5b = 15