given
[tex] \frac{f(x + h) - fx}{h} [/tex]
substitute the value given into the equation
[tex] = \frac{5(x + h) ^{2} }{h} - 5 {x}^{2} [/tex]
[tex] = 5( {x}^{2} + 2xh + {h} ^{2} ) - 5 {x}^{2} [/tex]
thus simplifying the numerator only
[tex] = \frac{5 {x }^{2} + 10xh + 5 {h}^{2} - 5 {x}^{2} } {h} [/tex]
[tex] = \frac{10xh + 5 {h}^{2} }{h} [/tex]
[tex] = 10x + 5h[/tex]
