Note: there are easier ways to solve this ...but since the quadratic formula was requested, here goes:
Remember the quadratic formula tells us that given a quadratic in the form:
#color(white)("XXX")color(red)ax^2+color(blue)bx+color(green)c=0#
the solutions are given by the formula
#color(white)("XXX")x=(-color(blue)b+-sqrt(color(blue)b^2-4color(red)acolor(green)c))/(2color(red)a)#
Converting the given equation: #3x^2-15x=0# into this form:
#color(white)("XXX")color(red)3x^2+color(blue)(""(-15))x+color(green)0=0#
The solutions are
#color(white)("XXX")x=(-color(blue)(""(-15))+-sqrt((color(blue)(-15)^2-4 * color(red)3 * color(green)0)))/(2 * color(red)3)#
#color(white)("XXX")=(15+-sqrt(15^2))/6#
#color(white)("XXX")x=30/6=5color(white)("XX")orcolor(white)("XX")x=0#
