Answer: 21
steps:
1) 6(4) - 3
2) 24 - 3
3) 21
Solution:
Let me define pyramid first.A pyramid is a Polyhedron , whose base can be any polygon and it's faces are triangular which meet at a point called vertex.
Volume of Pyramid = 
Side of base which is an equilateral triangle= 4 cm
Area of whole pyramid = 12 square cm
Area of Equilateral triangle having side 4 cm=
cm²
So, Area of three triangle which are faces of pyramid = (12 - 4 √3)cm²
Area of triangle = 
12 - 4 √3= 
Height =
Height=(6-2√3) cm
Volume of pyramid = Area of Equilateral triangle which is it's Base × Height of Pyramid
=4√3 × (6-2√3) cm³
=(24 √3 -24)
= 24 (1.732-1)
=24 × 0.732
=17.568 cm³
X² + 12x = 40
x² + 12x + (6)² = 40 + (6)²
x² + 12x + 36 = 40 + 36
(x + 6)² = 76
Answer: 36 must be added to both sides to complete the square.