Hello, please consider the following.
x is obviously different from 0 and then, dividing by x is legit.
For the first 105 miles the speed is x, so the time spent is
[tex]\dfrac{105}{x}[/tex]
For the second part, the speed is 1.4 x, so the time spent is
[tex]\dfrac{105}{1.4x}=\dfrac{75}{x}[/tex]
In total, the time she spent driving is
[tex]\dfrac{105}{x}+\dfrac{75}{x}\large \boxed{=\dfrac{180}{x}}[/tex]
Thank you.
