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4x + 5a + 4y + 5b

                                   Numbers can be added in any order, so we can rearrange the expression.

=  4x + 4y + 5a + 5b

                                   Now we can factor  4  out of the first two terms and  5  out of the last two terms.

=  4(x + y) + 5(a + b)   Notice that if we distributed the  4  and the  5 , we would get the last expression.

                                   Since  x + y  =  -2 ,  we can substitute  -2  in for  x + y .

=  4( -2 ) + 5(a + b)

                                   Since  a + b  =  -2 ,  we can substitute  -2  in for  a + b .

=  4( -2 ) + 5( -2 )

=  -8  +  -10

=  -18