Given that the triangle is an isosceles right angled triangle, then the value of x will obtain using the Pythagorean theorem, given by:
c^2=a^2+b^2
where
c is the hypotenuse, a and b are the legs
thus
x^2=12^2+12^2
x^2=144+144
x^2=288
hence
x=√288
x=√(2×144)
x=2√12
