Take the square root of 324 to find the side length of the square.
√324 = 18cm
Triangle area formula: A = bh/2
b = 18
h = 40
Solve using the given values.
A = 18*40/2
A = 720/2
A = 360cm^2
Multiply by 4 since there are four triangles.
360 * 4 = 1,440cm^2
Add to the area of the square to find the total surface area.
1,440 + 324 = 1,724cm^2
Best of Luck!