What is the domain of the function y=10-5x

Any real number (-inf, inf)

Domain of definition of a function is the set of numbers which the variable attains and for which the function is defined.

f (x) = sqrt (5x-10)

here x can have value equal to any real number >=2 because if x attains value less that 2, 5x-10 becomes negative and sqrt (5x-10) has no real value.

therefore the domain of the function f (x) is (2, infinity) inclusive of 2.