Remember that
In any triangle: The shortest side is always opposite the smallest interior angle. The longest side is always opposite the largest interior angle
so
In this problem
The largest angle is opposite the 35 units side
therefore
Applying the law of cosines
[tex]c^2=a^2+b^2-2abcosC[/tex]where
c=35 ft
a=25 ft
b=15 ft
C is the largest angle (angle between side a and side b)
substitute
[tex]35^2=25^2+15^2-2(25)(15)cosC[/tex]Solve for cosC
[tex]cosC=\frac{25^2+15^2-35^2}{2(25)(15)}[/tex][tex]C=120^o[/tex]