Exercise 4 Page 523

The expression $b\perp c$ says that $b$ and $c$ are perpendicular. Let's draw the diagram and then we can reason about it.

According to the Lines Perpendicular to a Transversal Theorem, in a plane, if two lines are perpendicular to the same line, then they are parallel to each other. From the diagram, we see that $c$ and $d$ are both perpendicular to $a.$ Therefore we can say that $c\parallel d.$

Next, according to the Perpendicular Transversal Theorem, in a plane, if a transversal is perpendicular to one of two parallel lines, then it's perpendicular to the other line. Therefore, we can finally say that $b\perp c.$