The area of one of the triangular lateral faces is
[tex]\dfrac{401.48-166.28}6=39.2[/tex]
You're told that the slant height, which is the same as the height of the triangular face, is 9.8, so you have
[tex]39.2=\dfrac12\times9.8\times b[/tex]
where [tex]b[/tex] is the length of the base of the triangle, which is also the same as the side length of the base of the pyramid. So
[tex]b=8[/tex]
