Backward error of solving $Ax=b$ with perturbed matrix $A$

Question

Let $A=\begin{bmatrix} 1 &-0.55 \\ -2 & 1.06 \end{bmatrix}, b=\begin{bmatrix} 1\\ -1 \end{bmatrix}$. How to compute the backward error of the algorithm of solving $Ax=b$, with respect to perturbation in the matrix? Here I perturb matrix $A$ to get $A'=\begin{bmatrix} 1+\delta &-0.55 \\ -2 & 1.06 \end{bmatrix}$, where $0<\delta<1$.