Line [i]b[/i] was created to be perpendicular to line [i]a[/i] and line [i]c[/i] was created to be perpendicular to line [i]a[/i]. So in other words, both line [i]b[/i] and [i]c[/i] are perpendicular to the same line.
What do you think is true about lines [i]b[/i] and [i]c[/i]?