.
To answer the question, you need to solve this equation
h(t) = 0
(saying that the phone reached the level h= 0 of the ground), and find "t" from there.
This equation is
61 - 6t - 5t^2 = 0,
or, EQUIVALENTLY,
5t^2 + 6t - 61 = 0.
Next, such problems the teachers give to students AFTER they learned the quadratic formula for solving quadratic equations.
So, I assume that you know how to do it.
If, in opposite, due to some reason, you are unfamiliar with it, you may learn it from the lessons
- Introduction into Quadratic Equations
- PROOF of quadratic formula by completing the square
in this site.
To see many other similar (and different) solved problems, look into the lessons
- Problem on a projectile moving vertically up and down
- Problem on an arrow shot vertically upward
- Problem on a ball thrown vertically up from the top of a tower
- Problem on a toy rocket launched vertically up from a tall platform
in this site.
Consider these lessons as your textbook, handbook, tutorials and (free of charge) home teacher.
Read them attentively and learn how to solve this type of problems once and for all.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this textbook under the topic "Projectiles launched/thrown and moving vertically up and dawn".
Save the link to this online textbook together with its description
Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson
to your archive and use it when it is needed.
If after my post you still have questions, then let me know.