Question

work is scalar quantity so it can be negative but how and why

Answer 1

scalar quantity is the one which has magnitude only . It can be positive, negative or zero.
e.g. -1,0 ,+1 etc

Answer 2

scalar quantity is used only for magnitude and magnitude means size.
now work is an example of scalar quantity because power apply with magnitude work is done only with power applied on an object it does not depend on any direction it depends on size does work is an example of scalar quantity.