Substituting, we have
... f(x+4) = (x+4)² + 8(x+4) +7
... = x² +8x +16 +8x +32 +7
... = x² + 16x + 55
Subtracting the original, we have
... f(x+4) - f(x) = (x² +16x +55) -(x² +8x +7)
... f(x+4) - f(x) = 8x +48
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