In this picture, the fact that PQR and PML are similar triangles is important. This means that one triangle can be converted into the other by a scale factor. Among others, this means that
PQ/PR = PM/PL (common property of similar triangles)
Filling in what we know, we get:
(16x+2)/70 = 21/15, which simplifies to
8x+1 = (21*70)/(2*15) = 49
x = 48/8 = 6
