\(\left(x+15\right):3=7\)
\(x+15=7.3\)
\(x+15=21\)
\(x=21-15\)
\(x=6\)
_____
\(5x-3x-12=8\)
\(\Leftrightarrow2x-12=8\)
\(2x=8+12\)
\(2x=20\)
\(x=20:2\)
\(x=10\)
______
\(4.\left(6-x\right)=280:36\)
\(4.\left(6-x\right)=\dfrac{280}{36}\)
\(6-x=\dfrac{280}{36}.\dfrac{1}{4}\)
\(6-x=\dfrac{35}{18}\)
\(x=6-\dfrac{35}{18}\)
\(x=\dfrac{108}{18}-\dfrac{36}{18}\)
\(x=\dfrac{73}{18}\)