$\displaystyle \begin{array}{{>{\displaystyle}l}} Do\ 0< a< \pi \Rightarrow sina< 0\ or\ sina >0\ \\ sin\ a=\pm \sqrt{1-cos^{2} a} =-\sqrt{1-\frac{4}{25}} =\pm \frac{\sqrt{21}}{5}\\ sin\left(\frac{\pi }{4} -a\right) =sin\frac{\pi }{4} cosa-cos\frac{\pi }{4} .sina=\frac{\pm \sqrt{42} +2\sqrt{2}}{10}\\ cos\left(\frac{\pi }{4} -a\right) =cosa.cos\frac{\pi }{4} +sina.sin\frac{\pi }{4} =\frac{\mp \sqrt{42} +2\sqrt{2}}{10}\\ tan\ \left(\frac{\pi }{4} -a\right) =\frac{sin\left(\frac{\pi }{4} -a\right)}{cos\left(\frac{\pi }{4} -a\right)} =-\frac{25+4\sqrt{21}}{17} \end{array}$
