Step 1:
Draw a pair of perpendicular lines and name the point where they meet as A.
Step 2:
Mark B on one of the lines, 8 cm from A and mark C on the other line, 4 cm from A. [1 mark]
Step 3:
Join the points B and C to get the required right angles triangle.
From the above construction, we see that the length of side BC is 8.9 cm
Then the perimeter of this triangle is
4 cm + 8 cm + 8.9 cm = 20.9 cm
[1 mark]