In the figure given, two circles with centres `C_1 and C_2`, are 35 units apart,i.e. `C_1C_2 = 35`. The radii of the circles with centres `C_1 and C_2`, are 12 and 9 respectively. If P is the intersection of `C_1C_2`, and a common internal tangent to the circles, then `l(C_1P)` equals