Hello,
\(r=2\\
s=-15\\
u_{1}=28\\
u_{n}=r*u_{n-1} +s\\
u_{2}=r*u_{1}+s\\
u_{3}=r*u_{2}+s=r^2*u_{1}+r*s+s\\
u_{4}=r*u_{3}+s\\
=r^3*u_{1}+r^{2}*s+r*s+s\\
=r^3*u_{1}+s(r^2+r+1)=r^3*u_{1}+s* \frac{r^3-1}{r-1} \\
...\\
u_{n}=r*u_{n-1}+s\\
\ =(r^{n-1}*u_{1}+s* \frac{r^{n-1}-1}{r-1})*r+s \\
\ =r^{n}*u_{1}+s* \frac{r^n-r+r-1}{r-1} \\
\boxed{u_{n}=r^{n}*u_{1}+s* \dfrac{r^n-1}{r-1}} \\ \\
\ u_{56}=2^{56}*28-15* \frac{2^{56}-1}{2-1} =936748722493063183 \\\)
