Area of a rectangle is length times width. A square is the case where those two sides match, so the area is a side squared. The ACT does not give a formula sheet. Area as a set is in area and volume.
Read the labels
An 8-by-5 rectangle has area 40. If the figure shows a diagonal, that is not a side. If it shows a frame around a garden, you may need the inner rectangle, not the outer one. Units stay squared: square feet, not feet.
A parallelogram uses base times perpendicular height, in area of a parallelogram. Surface area of a box is a different job, in surface area. The short list of formulas is in ACT Math formulas.
The bottom line
Length times width. Height in a leaning figure is not a slanted side.
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This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.