Graph y = k as a horizontal line and click where it meets your curve. Those x-values are the solutions to f(x) = k. It is the same intersection move as a system. Systems in Desmos are in systems in Desmos.
When the question is “for which x does y equal 5”
Type the function, then type y = 5. Click each intersection. A quadratic can meet the line twice. A line meets it once unless they are parallel. Evaluating at a known x is the opposite direction, in evaluating a function at a point.
If nothing shows, the intersection is off the window. Zoom out, in zooming off-screen graphs. Desmos itself is in Desmos on the ACT.
The bottom line
Horizontal line at the target value, then read the x-coordinates of the crossings.
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.