my math program gave me a question i thought i knew how to answer, but i keep getting it wrong, and nobody can tell me what im doing wrong.

in an equilateral triangle where all sides = 7, find the area.

so i cut it in half, so A = 3.5 and C =7. so, the squares are 12.25 and 49.

to find B, i did 49 - 12.25 = 36.75 , and then got the square root of that, 6.062. so, that is the hight of the triangle. 7 * 6.062 is 42.434. half of that is 21.217.

i put that as the answer, but got it wrong, nobody can explain to me why, ive tried rounding it to 21, 21.2, and 21.22..and none of those helped. can somebody help me with a different way of doing this, or tell me what im doing wrong?