One of the function types that you may see on the New SAT is the exponential function, which students are normally introduced to in Algebra II. These functions, which can be used to model population growth, to compute compound interest, and for other applications in science, have equations in the form:
f(x) = a·b^x
In this equation, a and b are constants and the variable, x, appears as an exponent.
The number a is the initial value (when x = 0). For a population growth problem, it would represent the initial population of whatever group you are looking at. For a compound interest problem, the a represents the amount of the deposit or the loan.