1) a/b=3/9 => a/b=1/3 => 3a=b
a-b=12 => a-3a=12 => -2a=12 => a= -6
3(-6)=b => b= -18
2) a/b=5/10 => 5a=b
a+b=75 => a+5a=75 => 6a=75 => a=12,5
b= 5•12,5 => b=62,5
3)a-b=50 => a=50+b
a/b =2/7 => 7a=2b => 7(50+b)=2b => 350 +7b=2b =>5b= -350 =>b= -70
a=50-70 => a= -20