See the pic below ! and show work!

Well I don't know !
Let's check out the choices:

A).    3a - 4b = 21,  a=0, b=7

         3(0) - 4(7) = 21

             0  -  28  = 21    Nope.  That's not true.

B).  3a - 4b = 21,  a=-3,  b=2

     3(-3) - 4(-2) = 21

       -9    +  8  = 21

             -1    = 21      Nope.  That's not right.

C).  3a - 4b = 21,    a=-2,  b=-3

      3(-2) - 4(-3) = 21

         -6  + 12  = 21

                 6  =  21          Nope.  That's not it.

Only one choice left.
This one had better be it:

D).  3a - 4b = 21,    a=7,  b=0

       3(7) - 4(0) = 21

         21 -  0    = 21

               21  =  21            Yay !   That's it !
 

It is d because 3•7= 21
And 4•0= 0
21+0= 21

Gggģcawrygr3ezt9d hu and the other countries have been working to make it easier to box up their homes in an area where the government has not yet decided which countries will have the option of the government