Every parallelogram area question is the same two moves: find the base and the perpendicular height, then multiply. That's it.
The Two Moves
One — read off the base and the perpendicular height. The base is any one side; the height is the distance straight across to the opposite side, measured at a right angle. In a question it's usually the dashed line, often with a little right-angle square on it.
Two — multiply. Area = base × height. Write it, drop the numbers in, multiply.
A Worked One
A parallelogram with a base of 8 cm and a perpendicular height of 5 cm: 8 × 5 = 40, so the area is 40 cm². Notice we never touched the slanted sides — they don't come into it.
Decimals Are No Different
If the height is 4.5 m and the base is 6 m, the method doesn't change: 6 × 4.5 = 27 m². Multiply carefully and keep the "square" on the units.