Answer
Please see the work below.
Work Step by Step
We know that
$a=\frac{2y}{t^2}$
We plug in the known values to obtain:
$a=\frac{2(10.8)}{(3.00)^2}=2.40\frac{m}{s^2}$
Now we can find the force
$F=ma=2.50(2.40)=6.00N$
As $F=F_1+F_2$
$\implies F_2=F-F_1$
We plug in the known values to obtain:
$F_2=6.00-15=-9.00jN$
