[tex]\displaystyle\bf\\1)\\\\f(x)=5x^{2} -x\\\\f(1)=5\cdot 1^{2} -1=5-1=4\\\\\boxed{f(1)=4}\\\\2)\\\\y=5x+4\\\\y=-1\\\\-1=5x+4\\\\5x=-5\\\\\boxed{x=-1}\\\\3)\\\\f(x)=x^{2} -25\\\\f(x)=0\\\\x^{2} -25=0\\\\(x-5)(x+5)=0\\\\x-5=0 \ \ \ \Rightarrow \ \ \ x_{1} =5\\\\x+5=0 \ \ \ \Rightarrow \ \ \ x_{2} =-5\\\\Otvet \ : \ \boxed{5}[/tex]