Area of a parallelogram is base times perpendicular height, not base times a slanted side. A base of 9 and a height of 4 give 36. Area as a set is in area and volume.
Height is the drop, not the lean
The figure often labels both the slanted side and a dashed perpendicular. Use the dashed one. Using the slanted side produces a number the test writers already put in the choices.
A rectangle is the special case where height equals a side, in area of a rectangle. Circles use a different formula, in circle problems. The short list is in ACT Math formulas.
The bottom line
Base times perpendicular height. The slanted side is a trap.
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