Area of a triangle is 1/2 × base × height. So area = 1/2 × 58 × 10 = 290 square mm. The height must be perpendicular to the base. A triangle is half of a rectangle or parallelogram with the same base and perpendicular height, so we divide by 2. Always use the perpendicular height, not a slant side, when calculating area. So the correct choice is 290 mm².