So for the surface area (SA) of the pyramid,
SA = 4×1/2×b×h, base (b) is side length and h is the unknown slant height. shortened is:
SA = 2bh
140.4 = 2×9 h
140.4 = 18h
h = 140.4/18 = 7.8 cm
**there's another shorter way to handle equilateral triangles, if you've learned the secret to a 30-60-90 right triangle ;)
