Question 1086844
Rate x Time = Distance,
{{{P=M+7}}}
{{{D[P]=(M+7)3=3M+21}}}
{{{D[M]=M(3)=3M}}}
and
{{{D[P]^2+D[M]^2=39^2}}}
Substituting,
{{{(3M+21)^2+(3M)^2=1521}}}
{{{9M^2+126M+441+9M^2=1521}}}
{{{18M^2+126M-1080=0}}}
{{{M^2+7M-60=0}}}
{{{(M+12)(M-5)=0}}}
Only positive rates make sense here,
{{{M-5=0}}}
{{{M=5}}}{{{kph}}}
and
{{{P=5+7}}}
{{{P=12}}}{{{kph}}}