Question

calculeaza : a - c stiind ca a + b = 21 si b+ c =7

Answer 1

$- \left \{ {{a+b=21}\atop{b+c=7}} \right. \\ \\ a-c = 21-7 =\ \textgreater \ a-c=14$

Answer 2

a+b=21 
b+c=7
a-c=?
a-c=21-7
a-c=14