If 1=square foot - 1, what is the value of i^3

I think you meant to say "root" instead of "foot". Also, I think you meant to use the letter i instead of the number 1 on the left side of the equation. It should be this:

i = square root of - 1 = sqrt (-1)

Often it's handier to use the shortcut "sqrt" for "square root"

Let's square both sides of the equation given:

i = sqrt (-1)

i^2 = (sqrt (-1)) ^2

i^2 = - 1 ... notice squaring the square root have the two cancel out and go away

Now multiply both sides by i

i^2 = - 1

i^2*i = - 1*i

i^2*i^1 = - i

i^ (2+1) = - i

i^3 = - i

which is the final answer