part a
when the question states perimeter, it means the total length.
the question have given you the length of each side. so, you just need to add all of them to find the perimeter.
Perimeter=a+b+c+d
=x+7+(2x-8)+8x+(x+10)
=x+7+2x-8+8x+x+10
=12x+9
part b
the question now states what is the value of x which is x=4, thus, you subtitute x=4 into the answer that you got in part a.
12(4)+9=57