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.