A rectangle is the honest one: $A = l \cdot w$, length times width, done. Tip a rectangle sideways and you get a parallelogram — same idea, one catch: $A = b \cdot h$ (A equals b times h), where $h$ is the perpendicular height — the straight-across distance between the base and its opposite side, never the slanted side you could walk along.

Exam papers love to hand you that slant. It always takes the scenic route, so it is always LONGER than the true height, and using it always overshoots. When a leaning shape offers two different “heights”, take the one that meets the base at a right angle — the little square drawn in a diagram's corner is the formula pointing at its own ingredient.