You have to cross multiply (multiply the numerator of the first by the denominator of the second and the numerator of the second by the denominator of the first).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[19]
\(\frac{x+4}{3} = \frac{4}{10}\)
\(10x+40=12\)
\(10x = -28\)
\(x=-2.8\)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[21]
\(\frac{4}{n+8} = \frac{10}{8}\)
\(32 = 8n + 64\)
\(8n = -32\)
\(n=-4\)
