Rainbow Spiral

This is known as the Fibonacci Sequence:

$1, 1, 2, 3, 5,\ldots, F_n = F_{n-1}+F_{n-2}$

$x= 21+34=\boxed{55}$

But @Zandra Vinegar , I used to think that $\displaystyle x=\sqrt{55}$ because it's on the side of the square. Please place $x$ in the middle. of the square.

fibonacci series....just multiply the previous number on the series by the "golden ratio = 1.616..........."

We know that $x = 21 + 34 = \boxed{55}$ by looking at the $2$ squares on the left of it.

The spiral is kind of a too obvious hint that this is the Fibonacci sequence... so 55 is the next number.