be our mirror line. to the right of the mirror line, draw a dot and call it a. from this dot, draw a line perpendicular to the mirror line. measure the distance from the dot to the mirror line using this perpendicular line you just drew. in our case, it comes out to be two boxes or two units long.
that's all you need to know. every point on one side of the mirror line will have the same distance from the mirror line on the other side. this means we need to measure two box lengths away from the mirror line on the opposite side and draw a dot there 