b = 4√3
using Pythagoras' identity in the given triangle
c² = a² + b² ( substitute c = 2b and a = 12 into the equation )
(2b)² = 12² + b²
4b² = 144 + b² ( subtract b² from both sides )
4b² - b² = 144
3b² = 144 ( divide both sides by 3 )
b² = 48 ( take the square root of both sides )
b = √48 = √( 16 × 3 ) = √16 × √3 = 4√3 ' exact value '
