When you write a function in the form [tex] y=f(x) [/tex], y is the dependent variable, and x is the independent variable.
So, if b is the independent variable, it means that we have to solve the expression for r.
To do so, we perform the following steps: start with
[tex] 16b = 4r - 12 [/tex]
Add 12 to both sides
[tex] 16b+12 = 4r [/tex]
Divide both sides by 4:
[tex] 4b+3 = r [/tex]
So, we wrote the expression in the form [tex] r = f(b) = 4b+3 [/tex], which means that r is the dependent variable and b is the independent variable, as requested.