This resource from the Mathematics Assessment Project was found at OER Commons. https://www.oercommons.org/courses/finding-equations-of-parallel-and-perpendicular-lines/view. This lesson unit is intended to help teachers assess how well students are able to understand the relationship between the slopes of parallel and perpendicular lines and, in particular, to help identify students who find it difficult to: find, from their equations, lines that are parallel and perpendicular; and identify and use intercepts. It also aims to encourage discussion on some common misconceptions about equations of lines.

Students must already be comfortable graphing lines, find slope from points and from an equation, and identifying x and y intercepts from an equation and from a line.

This Quizizz (https://quizizz.com/admin/quiz/5b1ffb01e6ea68001966a667/parallel-and-perpendicular-lines) created by akmartin can be used as extra practice identifying slopes of parallel and perpendicular lines.

I would encourage students to use Desmos (https://www.desmos.com/testing/virginia/graphing) to help them answer questions and check their work graphically.