The formula for calculating the distance between two points is:
#d = sqrt((color(red)(x_2) - color(blue)(x_1))^2 + (color(red)(y_2) - color(blue)(y_1))^2 + (color(red)(z_2) - color(blue)(z_1))^2)#
Substituting the values from the points in the problem gives:
#d = sqrt((color(red)(12) - color(blue)(3))^2 + (color(red)(-11) - color(blue)(-4))^2 + (color(red)(6) - color(blue)(15))^2)#
#d = sqrt((color(red)(12) - color(blue)(3))^2 + (color(red)(-11) + color(blue)(4))^2 + (color(red)(6) - color(blue)(15))^2)#
#d = sqrt(9^2 + (-7)^2 + (-9)^2)#
#d = sqrt(81 + 49 + 81)#
#d = sqrt(211)#
Or
#d = 14.526# rounded to the nearest thousandth.