ANSWER
648 square units.
EXPLANATION
The surface area of the regular pyramid is the area of the base plus the area of the 4 triangular faces.
We use the formula;
[tex]S.A = {l}^{2} + 4 \times \frac{1}{2} bh[/tex]
where l=12 units is the length of the square base and h=21 units is the vertical height of the triangular faces.
We substitute the values to get;
[tex]S.A = 12^{2} + 4 \times \frac{1}{2} \times 12 \times 21[/tex]
[tex]S.A = 144+ 504[/tex]
[tex]S.A =648 {units}^{2} [/tex]
