To find this answer you will use the formula for finding the area of the triangle because that shape is a triangle.
A = 1/2 bh
198 = 1/2 x b x 12
198 = 6b
b = 33 cm
We know the total area and we know the slant height is 12 cm so we substitute these in and solve for h.
The length of the base edge is 33 cm. You should substitute this back in to check.
1/2 x 33 x 12 = 198 square cm.