You have added -13 to every function value, so every point on the graph is 13 "spaces" lower. The y-intercept is one such point, so ...
-The y-intercept is 13 spaces lower.
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<em>Comment on notation</em>
In plain text an exponent is preceded by a caret (^). If the exponent has arithmetic involved (addition, subtraction, multiplication, division), then the entire exponent needs to be enclosed in parentheses. When an exponential term that has a constant base is multiplied by another constant, then some sort of multiplication indicator is useful. Your expressions might be written ...
... f(x) = 4·2^x
... f(x) = 4·2^x - 13 . . . . . . or, better, f(x) = 4(2^x) - 13 . . so it is clear -13 is not part of the exponent
