To calculate x using Pythagoras theorem, we need to use the formula:

c^2 = a^2 + b^2

In this case, a = 12 and c = 15. We want to find b, which is represented by x in the equation.

Plugging in the values, we have:

15^2 = 12^2 + x^2

225 = 144 + x^2

Subtracting 144 from both sides:

81 = x^2

Taking the square root of both sides:

x = ±9

Therefore, x can be either 9 or -9.