Math!
posted by Vince .
A rectangle is 15cm x 12cm. If the length is reduced by R cm and its width is increased by R cm. Its area is reduced by 4cm^2.

I assume you are looking for the value of R
Original rectangle has area 15x12 or 180 cm^2
new length = 15R
new width = 12+R
so 180  (15R)(12+R) = 4
solve for R