Verified answer

(-12, 6)=a1(2, -4)+a2(-5, 1)

-12 = 2a1-5a2

6 =-4a1+a2

a2=6+4a1

-12=2a1-5(6+4a1)

-12=2a1-30-20a1

30-12=-18a1

-18a1=18

a1=-1

a2=6+4a1=6-4 =2

-1 e 2