There is an easier way to do this. We know the radius is 170/2 or 85 Lets create a right triangle by drawing a radius from the center fo the circle to the point where the chord intersects the circle and a Perpendicular line from the center of the circle to the chord ( this is the side we will have to solve for). This perpendicular line will bisect the chord so this length will be 26/2= 13
DRAW A PICTURE
Now we can us Pythagorean theorm x^2 + (13)^2 = (85)^2
x^2 +169 = 7225
x^2 =7056. Take the square root of both sides
x=84 There you go