Since this is a right triangle, you use pythagorean theorem (a^2 + b^2 = c^2, with a and b being sides and c being the hypotenuse. Here, the hypotenuse = 3 and one of the side lengths is 2 and the other is z, so your equation should be 2^2 + z^2 = 3^2 (you can switch up whether the 2 or z goes first). Then, simplify so your equation should be 4 + z^2 = 9. Subtract 4 from both sides to get z^2 = 5. Square root both sides. Your answer should be the square root of 5. Since it is asking for a decimal, your answer should be 2.2 (rounded to the tenths place).