The answer is
First of all, note that we can solve all angles, since we have
![\hat{A}+\hat{B}+\hat{C} = 180 \iff \hat{A}+90+44.25 = 180 \iff \hat{A} = 180-90-44.25 = 45.75](https://img.qammunity.org/2019/formulas/mathematics/middle-school/7s7px2wh98aywzq9skr3ifkkk583gp1i05.png)
where
is the angle centered in vertex A, and so on.
Now we can use the law of sines, which states that the ratio between a side and the sine of the opposite angles is constant.
So, you would have
![\frac{AC}{\sin(\hat{B})} = \frac{BC}{\sin(\hat{A})}](https://img.qammunity.org/2019/formulas/mathematics/middle-school/kva3ox8th05ba299sjnt8sf5ha23cp2txt.png)
plug in the known values:
![(8.6)/(\sin(90)) = (BC)/(\sin(45.75))](https://img.qammunity.org/2019/formulas/mathematics/middle-school/toga1mk2ipvha5rev32qg3nbpq20882udi.png)
Since sin(90)=1, the denominator of the first fraction disappears. Finally, we can solve for BC:
![BC = 8.6 \cdot \sin(45.75) \approx 8.4338\ldots](https://img.qammunity.org/2019/formulas/mathematics/middle-school/hm0tkolz7mt0id3w3gk245hq1bewni7pu8.png)
Which gives 8.43 when rounded as required.