Hello from MrBillDoesMath!
Answer:
sqrt(6)
Discussion:
The triangle, while not indicated, appears to be a right triangle (i.e. has a 90 degree angle). If so, from the Pythagorean theorem,
x^2 + 3^2 = ( sqrt(15) ) ^ 2 =>
x^2 + 9 = 15 => ( subtract 9 from both sides)
x^2 = 15 - 9 = 6 => (take the square root of both sides)
x= sqrt(6)
Regards,
MrB
