If P(x,y),F1(3,0), F2(-3,0) a nd 16x^2+25y^2=400, then find PF1+PF2.
Hint: an ellipse is the locus of points whose distances from the two foci have a constant sum. That sum is the length of the major axis.
So, just write your equation in standard form for an ellipse, and you can read off the value of the semi-axes.