First, add #color(red)(6)# to each side of the equation to isolate the #x# term while keeping the equation balanced:
#x^2/15 - 6 + color(red)(6) = -2 + color(red)(6)#
#x^2/15 - 0 = 4#
#x^2/15 = 4#
Next, multiply each side of the equation by #color(red)(15)# to isolate the #x^2# while keeping the equation balanced:
#color(red)(15) xx x^2/15 = color(red)(15) xx 4#
#cancel(color(red)(15)) xx x^2/color(red)(cancel(color(black)(15))) = 4 xx 15#
#x^2 = 4 xx 15#
Now, take the square root of each side of the equation to solve for #x# while keeping the equation balanced. Remember, the square root of a number produces both a positive and a negative result:
#sqrt(x^2) = +-sqrt(4 xx 15)#
#x = +-sqrt(4)sqrt(15)#
#x = +-2sqrt(15)#