Question 845527
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Hi,
{{{(matrix(2,2,a[1],b[1],a[2],b[2]))}}}= {{{a[1]b[2]-a[2]b[1]}}}
Yes D = 31 = 21+10
{{{D[x] = 7*7 + 8 = 57}}}          x = 57/31
{{{D[y] = 3*4 -5*7 = -23}}}        y = -23/31
CHECKING our answer***
 3x-2y= 7
{{{3(57/31) - 2(-23/31) = 171/31 + 46/31 = 217/31 = 7}}} 
Wish You the Best in your Studies.