[tex]x = 7 - p\implies x + p = 7\implies \boxed{p = 7 - x} \\\\[-0.35em] ~\dotfill\\\\ y = 3p+4\implies y = 3(7-x)+4\implies y = 21-3x+4\implies \blacktriangleright y = 25 - 3x\blacktriangleleft[/tex]
