Step 1: Draw a line segment BC = 7 cm.
Step 2: Draw an arc from point B cutting BC at N. (Choose any radius.)
Step 3: Keep compass at point N with same radius selected in step 2, cut the previous arc at M.
Step 4: Join line segment BM.
Step 5: Produce BM to any point P
Step 6: Cut BR = 12 cm, from BP.
Step 7: Join CR.
Step 8: Draw a perpendicular bisector of RC which intersects BR at A.
Step 9: Join line segment AC.
Hence, △ABC is the required triangle.