Let's go:
[tex]\frac{42}{6x - 1} = \frac{78}{10x + 5} \\\\ 42.(10x + 5) = 78.(6x - 1) \\\ 420x + 210 = 468x - 78 \\\ 468x - 420x = 210 + 78 \\\ 48x = 288 \\\ x = 6[/tex]
I hope I helped you.
