Final answer:
To find f(A + 4) for the function f(x) = x^2+ 3x, replace x with A + 4 and simplify the expression to get A^2 + 11A + 28.
Step-by-step explanation:
When asked to find f(A + 4) for the function f(x) = x^2+ 3x, we need to substitute A + 4 in place of x. This yields:
f(A + 4) = (A + 4)^2 + 3(A + 4) = A^2 + 8A + 16 + 3A + 12 = A^2 + 11A + 28.
This is a quadratic function in terms of A. Thus, f(A + 4) is obtained by performing the arithmetic operations to expand and simplify the expression.