I'm having a bit of trouble with this problem.

A ball is thrown upwards and stays in the air for 3.65 seconds. What is the initial velocity?
(I use an acceleration of -10 m/s/s)

I got an answer of 18.25 m/s/s but I'm not sure that it's correct. I would love some help!

hf=hi+vi*t-4.9t^2

hi=hf=0 solve for vi.

I would NEVER use 10m/s^2, g is nowhere on EArth equal to that.

I am reminded of those who use 3 for PI.

To find the initial velocity of the ball, we can use the equation of motion for an object in free fall:

v = u + at

where:
v = final velocity (which is 0 m/s when the ball reaches its highest point)
u = initial velocity
a = acceleration (-10 m/s^2)
t = time (3.65 seconds)

So the equation becomes:

0 = u + (-10) * 3.65

Rearranging the equation to solve for u:

u = -(-10) * 3.65
u = 36.5 m/s

Therefore, the initial velocity of the ball is 36.5 m/s (upwards).

It seems there was an error in your calculation. Make sure to multiply the acceleration (-10 m/s^2) by the time (3.65 seconds) to get the correct result.