to find the x intercept, we need to replace y by 0, so we get
[tex]\begin{gathered} 21x+3\cdot0=-63 \\ 21x=-63 \\ x=\frac{-63}{21}=-3 \end{gathered}[/tex]to find the y-intercept we need to replace x by 0
[tex]\begin{gathered} 21\cdot0+3y=-63 \\ 3y=-63 \\ y=\frac{-63}{3}=-21 \end{gathered}[/tex]