Every single triangle area question is the same three moves. Get these into your bones and there's nothing left to be scared of.
The Three Moves
One — find the base and the height. Pick a side to be the base (any side will do — more on that later). The height is the straight-up distance from the opposite corner to that base, always at a right angle. In the toy they're already marked for you in teal and violet.
Two — write the formula and drop the numbers in. Area = ½ × base × height. Don't skip writing it — it stops silly slips.
Three — work it out. Multiply the base and height first, then halve it (or halve one of them first if that's easier).
A Worked One, Slowly
Base 10, height 4. Formula: ½ × 10 × 4. Multiply the base and height: 10 × 4 = 40. Halve it: 20. And because area measures the flat space covered, the units are square ones — so 20 cm² if those were centimetres.
Watch Your Units
Whatever the base and height are measured in, the answer is that unit squared. Metres give square metres (m²), centimetres give square centimetres (cm²). Saying the little “squared” out loud every time is a good habit — it's the difference between a fence and a paddock.