Let \( A(-1,1) \) and \( B(2,3) \) be two points and \( P \) be a variable point above the line \( AB \) such that the area of \( \triangle PAB \) is \( 10 \). If the locus of \( P \) is \( ax+by=15 \), then \( 5a+2b \) is:
\( 6 \)
\( -\dfrac{12}{5} \)
\( -\dfrac{6}{5} \)
\( 4 \)