What is the square root of negative one

It doesn't exist, basically, since 10 squared is 100, and -10 squared is also 100.

Some random mathematicians invented an imaginary number, i, which is the result of the square root of -1.

->  ->  10i

Sail A has base equal to b and Height equal to h. Since sail B has base and height x times that of A, the base of B is bx and the height is hx

Since your sail has a base and a height it can be a triangle, a square, a rectangle or a rhombus, but I believe most sails are either triangles or rectangle so let's explore those. That should be enough to see that the answer is the same for the shapes listed.

TRIANGLE
The area of a triangle is (1/2)(b)(h). This is the area of sail A. The area of sail B is (1/2)(bx)(hx) = . That makes the area of B times greater.

RECTANGLE
The area of a rectangle is (b)(h). This is the area of sail A. The area of sail B is (bx)(hx) = That makes the area of B times greater.