The pythagorean theorem tells us that in a right-angle triangle, the squares of the legs of the triangle add up to the square of the hypotenuse: a2 + b2 = c2. An isoceles triangle can be divided into two equal right triangles.
Your base is 42 inches, so half of that is 21 inches. If we draw an imaginary line from the angle between the 29 inch sides to the midway point on the base, this is the "height" of the triangle, and the area of a triangle is half the base times the height.
So the height can be calculated using the pythagorean formula: 212 + h2 = 292. Solving this for h gives us a height of 20 inches.
That makes the area 1/2 x 42 x 20. = 420 square inches.
Stephen La Rocque.>