The velocity started at $12$ m/s, and after $0.5$ second, it was $7.1.$
The difference is $12 - 7.1 = 4.9,$
and this is the amount of the decrease in velocity in $0.5$ second.
In general, if we look at the ball after $t$ seconds, and it is moving at velocity $v,$ the decrease in velocity since it started moving upward is
$12 - v.$
We are given that the amount of decrease in the vertical velocity of the ball is directly proportional to how long it has been moving upward.
Since the decrease is directly proportional to $t$ there must be a constant $k$
$$ 12 - v = kt. \tag1 $$
From the fact that the velocity was $v=7.1$ when the time was $t=0.5,$
by plugging those values into Equation $(1)$ we can solve for $k.$
Once we have $k,$ we can set $v = 0$ and solve for $t$ to find out the time when the ball has stopped moving upward.
Rather than speaking of the amount of decrease in velocity,
I think it is more usual to speak of the change in velocity
over a period of time.
So if the velocity starts at $v = 12$ at time $t = 0,$
we say that $12$ is the initial velocity, and we call this $v_0.$
If the velocity at a later time is $v,$ the change is
$v - v_0 = v - 12$; we always measure change by taking "final minus initial."
Since $v = 7.1$ when $t = 0.5,$ at that time the change in velocity has been
$$v - v_0 = 7.1 - 12 = -4.9.$$
It's a negative number because the velocity is decreasing and because a decrease is represented by a negative change.
Since the decrease is proportional to time, so is the change (which is just exactly the opposite value). So we have
$$ v - 12 = Kt. $$
You can proceed to solve this just like when we had a formula for the decrease; it just happens that now the value of $K$ will be negative.