Question

stiind ca a +b =5 si b+c=7 afla 2xa+5xb+3xc

Answer 1

a + b = 5
b + c = 7
2a + 5b + 3c = 2a + 2b + 3b + 3 c
2(a + b) + 3(b + c) = 2 * 5 + 3 * 7 = 10 + 21 = 31  

Answer 2

       
$2a+5b+3c=\\=2a+2b+3b+3c= \\ =2(a+b)+3(b+c) =\\=2*5+3*7=\\=10+21=\boxed{31}$