From area = 1/2 × base × height, height = 2 × area ÷ base = 2 × 40 ÷ 10 = 8 m. A triangle is half of a rectangle or parallelogram with the same base and height, so we multiply by 1/2. First write the triangle area formula, then undo the multiplication by 1/2 by multiplying the area by 2. After that, divide by the known base or height. This keeps the calculation organized and gives the missing length unit.