F4 Math - Polynomiais etc

The length and the width of a rectangle are x cm and y cm respectively.
a) The perimeter of the rectangle is 68 cm. Write down an equation in x and y.
ans: x + y = 34
b) Suppose its length is increased by 4cm and its width is decreased by 4cm.
　(i) Find, in terms of x and y, the new area of the rectangle.
　ans: [(x+4)(x-4)]cm^2
　(ii) Find its origina dimensions if the new area is reduced by 40cm^2.

I dunna how to solve question b(ii).
Please help me to solve them by clear steps n show the steps. Thanks : )