Finf (dy/dx)
y=(1+28x^3)f(x)+(6x)g(x)(x^4+5x+7)+(6x)g(x)(x^4+5x+7)-(e^x+7x^4)f(x)+3x^2g(x)(4x^3+5+7)/((x^4+5x+7)^2
Aside from the unbalanced parentheses, your posting appears to be an attempt at a derivative, but it's all mixed up and basically incomprehensible.
d/dx x^3g = 6xg + 3x^2g'
what is y, anyway? The e^x stuff appears in line 2, and it seems that you have some fraction involving (x^4+5x+7) in the denominator, but it's gobbledegook.
Are f and g defined somewhere?