equailateral means that base=40mm
solve for height
split the triangle in half with a line perpendicular to the base
now you have 2 right triangles with base 40/2=20 and hypotonuse 40
solve for height or the other side
use pythagorean theorem
a^2+b^2=c^2
20^2+b^2=40^2
400+b^2=1600
subtract 400 fromb oth sides
b^2=1200
square root boht sides
b=√1200=20√3
the height =20√3
area of triangle=1/2base times height
base=40
height=20√3
1/2 times 40 times 20√3=20 times 20√3=400√3
the area is 400√3 mm^2