A biologist is growing a population of bacteria for a study. The number of millions of bacteria in the population is modeled by a function, f(x), where x is the number of days since yesterday. The table shows values of f(x) for different values of x.
x f(x)
−2 3.25
−1 3.5
0 4
2 7
4 19

The function f(x) can be represented by the equation f(x)=ax+b. Enter the value for a and b in the equation f(x)=ax+b.