X=y-11
x-3y=1
solve this linear equation

x(x+4) = 21, or x²+4x-21 = 0

Explination: The area of a rectangle is given by multiplying the length and width of the rectangle.  We know that the length is 4 more than the width; if x represents the width, this means that the length is x+4.

The area would then be x(x+4).  We know that the area equals 21; this gives us the equation

x(x+4) = 21

Using the distributive property, we have

x(x)+x(4) = 21

x²+4x = 21

To write in standard form, subtract 21 from each side:

x²+4x-21 = 21-21

x²+4x-21 = 0