This type of problem can be decided by looking at whether the function is positive or negative in the vicinity of the zeros. There are three roots: -12, -6, and 2 at which the function is clearly 0. Look at the four intervals the x axis:
interval x<-12: function is negative (just test using a number <-12, say, -13)
interval (-12,-6): function is positive
interval (-6, 2): function is negative
interval x>2: positive
From the above you deduce that at the roots the function must be crossing the x-axis (as opposed to just touching it) because the function value changes its sign every time.
