To evaluate f(3) in Desmos, type the function, then type the function name at 3, or drop 3 into a table. You do not have to substitute by hand on the digital ACT. Desmos access is in Desmos on the ACT.
One value versus many
f(x) = 2x² − 1, then f(3) is a single number. Type f(3) on a new line after you define f. If the question wants several x-values, a table is faster, in Desmos tables. Function notation on paper is in function notation.
Graphing y = 3 and reading intersections is a different job, in graphing y equals a constant.
The bottom line
Define the function, then ask Desmos for f at the x you were given. Read the number. Do not re-expand the algebra unless the choices are expressions.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
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.