A=3x^2+9x I'm assuming is the equation for the area? with x being squared
L=3x
W=??
since we know the basic formula of area to be A=W*L, we can move it around to solve for W and get A/L=W. From here we can plug in the known values of A and L to solve for W.
A/L=W
(3x^2+9x)/(3x)=W
(3x(x+3))/3x=W
x+3=W
this gives you x+3=W, as both terms in the numerator contain 3x, which is the denominator.
specifically, I'd answer the questions like this.
1. To find the area of a rectangle, you must multiply the length times width
2. To find the width of a rectangle knowing the area and length, you must solve for the width using the formula for area by plugging in the known values for area and length
3. Work shown above
