Draw a horizontal line of any length x.

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.

It's equilateral.

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!