# How do you find the inverse of A=((18, -3), (-3, 15))?

To find the inverse ${A}^{-} 1$of a matrix $A$ which is $2 x 2$ the formula is
${\left[\left(\begin{matrix}a & b \\ c & d\end{matrix}\right)\right]}^{- 1} = \frac{1}{a \cdot d - b \cdot c} \cdot \left[\left(\begin{matrix}d & - b \\ - c & a\end{matrix}\right)\right]$
Just replace for $a = 18 , b = - 3 , c = - 3 , d = 15$ to get
${A}^{-} 1 = \frac{1}{87} \cdot \left(\begin{matrix}5 & 1 \\ 1 & 6\end{matrix}\right)$