How do you use Heron's formula to find the area of a triangle with sides of lengths #12 #, #4 #, and #7 #?
The Heron formula is
Now just replace the values in the formula.
There are better choices than Heron, but all lead to an imaginary area because these are not side lengths of a real triangle because they do not satisfy the triangle inequality.
Don't, use one of these instead:
The negative squared area shows this triangle doesn't satisfy the triangle inequality.