Syntax advice: Write the linear combination only in Maple syntax $--$ there should be no equals sign. Use p$1,$

$p2,p3$ for the polynomials $p_1,p_2,p_3.$

For example, if your response was $p_3=3p_1+2p_2$ then enter

$3***p1+2**p2$

Alternatively, if your response was $q=3p_1+2p_2-7p_3$ then enter

$3**p1+2**p2-7**p3$