\(\Leftrightarrow\) 3x+x=19-15
\(\Leftrightarrow\)4x=4
\(\Leftrightarrow\)4x.\(\frac{1}{4}\)=4.\(\frac{1}{4}\)
\(\Leftrightarrow\)x=1
3x+15 = -x + 19
\(\Leftrightarrow\)3x + x = 19 - 15
\(\Leftrightarrow\)4x=4
\(\Leftrightarrow\)x=4:4
\(\Leftrightarrow\)x=1
Vậy x=1
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