Matrices Problem

Algebra Level 2

If matrix $A=\left(\begin{array}{cc}a& b\\b& a \end{array}\right)$ satisfies

$A^2-10A+16I=0,$

where $b>0$ and $I=\left(\begin{array}{cc}1&0\\0&1 \end{array}\right),$ what is the value of $a-b?$