$7428+a=12000$

$a=12000-7428$

$a=4572$

$7428+4572=12000$

 

$b-1646=31847$

$b=31847+1646$

$b=33493$

$33493-1646=31847$

 

$26\cdot c=7930$

$c=7930:26$

$c=305$

$305\cdot 26=7930$

 

$y:40=20840$

$y=20840\cdot 40$

$y=833600$

$833600:40=20840$