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Work Shown:
f(x) - g(x) = [ f(x) ] - [ g(x) ]
f(x) - g(x) = [ 7x+15 ] - [ 5x-6 ]
f(x) - g(x) = 7x+15 - 5x+6
f(x) - g(x) = (7x-5x)+(15+6)
f(x) - g(x) = 2x+21
