To solve this problem you essentially just need to solve the Pythagorean Theorem.
The radius and the chord actually form a right triangle within the circle. The distance of the chord from the center is the "short side", the x value is the "long side", and the radius is the hypotenuse. However, the x value in the triangle only represents half of the total chord, therefore the value must be doubled once the initial equation is solved.
^2 means squared, btw.
x^2 + 0.7^2 = 2.5^2
x^2 = 2.5^2 - 0.7^2
x = sq rt of 5.76
x = 2.4
2.4 * 2 = 4.8
The answer is 4.8.