[tex] \huge \purple{\tt{Answer:}} [/tex]
In a rectangle , opposite sides are equal in length .
So ,
[tex] \sf {8x-1=31} \\\\ \implies \sf {8x=31+1=32} \\\\ \implies \sf{x={\dfrac{32}{8}}} \\\\ \implies \sf{x=4} [/tex]
