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An element has an atomic number of 71. What is it's mass?

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There's not a mathematical relationship between the atomic number and mass of an element. If you locate element number 71 on the periodic table you will find that it is lutetium and that it has an average atomic mass of 174.97 amu or grams per mole. The mass number of an atom is the sum of its protons and neutrons.

There are different isotopes of Lutetium. Isotopes are atoms of the same element that have different masses because they have different numbers of neutrons. All lutetium atoms have 71 protons. There are two naturally occcuring isotopes: Lutetium-175 with 104 neutrons and and lutetium-174 with 103 neutrons. Their approximate masses are therefore 175 and 174 respectively. The average mass is approximately 175 because 97.4% of the lutecium in nature is Lu-175. It's a weighted average based on the abundance of each isotope.

There are many non-naturally occcuring isotopes of lutetium that are short-lived nuclear decay products. They have masses ranging from 150 to 184.

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