Draw another line of length x off at 60 degrees
and another line of length x from the other end of the horizontal line at 70 degrees.
Join the corner with the 70 degree angle to the end of the other line.
Since 2 sides are x and the angle between the sides is 60 degrees,
this is a special case of isosceles triangle.
Now we have 4 lines of length x.
Hence the angles of the "thin" isosceles triangle are 10, 85, 85.
Therefore the largest angle is 85+60 degrees.
Try drawing it!