Can a linear function be continuous but not have a domain and range of all real numbers?

Equation for the calculation double 2 and then add 5

Tim's phone service charges $26.39 plus an additional $0.21 for each text message sent per month. if tim's phone bill was $31.64, which equation could be used to find how many text messages, x, tim sent last month?

What is the axis of symmetry of the function f(x)=-(x+ 9)(x-21)