Show that the distance from a point (xZERO,yZERO) to a line Ax+By+C=0 is given by

|AxZERO+ByZERO+C| / sqrt(A^2+B^2)

Why absolute value? I get that hes putting the points into the slope of the other one to get maybe a parallel line. but thats all I get. why the sqrt of (a^2+b^2) ?