Minor of an element a_ij = M_ij

a₁₁ = a, Minor of element a₁₁ = M₁₁ = d

Here removing 1^st row and 1^st column from the determinant we are left out with d so M₁₁ = d.

Similarly, finding other Minors of the determinant

a₁₂ = c, Minor of element a₁₂ = M₁₂ = b

a₂₁ = b, Minor of element a₂₁ = M₂₁ = c

a₂₂ = d, Minor of element a₂₂ = M₂₂ = a

Cofactor of an element a_ij, A_ij = (-1)^i+j × M_ij

A₁₁ = (-1)¹⁺¹ × M₁₁ = 1 × d = d

A₁₂ = (-1)¹⁺² × M₁₂ = (-1) × b = -b

A₂₁ = (-1)²⁺¹ × M₁₁ = (-1) × c = -c

A₂₂ = (-1)²⁺² × M₂₂ = 1 × a = a