As we know that perimeter of a shape is the sum of all it's boundaries. Also , let assume that the missing side be y . So ,here ;
[tex]{:\implies \quad \sf (4x-4)+(3x+1)+y=13x-8}[/tex]
[tex]{:\implies \quad \sf 4x-4+3x+1+y=13x-8}[/tex]
[tex]{:\implies \quad \sf y+7x-3=13x-8}[/tex]
[tex]{:\implies \quad \sf y=13x-8-7x+3}[/tex]
Collecting like terms :
[tex]{:\implies \quad \sf y=(13x-7x)+(-8+3)}[/tex]
[tex]{:\implies \quad \bf \therefore \quad \underline{\underline{y=6x-5}}}[/tex]
Hence , the length of Missing side is 6x-5
