Base = 2 × area ÷ height = 2 × 30 ÷ 10 = 6 cm. 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.