(I)"if the width of the enclosure is x m, find in terms of x, the lenght of the enclosure"
The shape is a rectangle but fencing is required only for 3 sides, 2 widths and 1 length. Question specifies that x m is a width, so we can give length any variable (y etc.) . Fencing is perimeter so it will be / 2x + y = 100 (100 being the avaible number of fencing) / . In terms of x means that you need to put y on its own => y = 100 - 2x
(II)"show that the area of enclosure is given by A = 100x-2x^2"
The area forumla is / Area = width * length ( A = xy ) / . You have an equation from (i), so just fill in the Area equation. A = x(100 - 2x) == 100x - 2x^2
(III)"Find the value of x that maximes the area of the enclosure"
(IV)"Find the maximum area of the enclosure"
You have the basic done from (ii). You need to use calculus to differentiate the area equation and find the maximum, not the minimum!, which can be found in your log/text book.