If a and b must be positive integers, what is the largest integer n such that 13a + 18b = n has no solutions?

For how answer my question this will be the last question can you answer in math subject

Given: circle o, cd is diameter, chord ab || cd. arc ab = 74°, find arc ca.

Abicycle training wheel has a radius of 3 inches. the bicycle wheel has a radius of 10 inches.approximately how much smaller, in square inches and rounded to the nearest hundredth, is the area of the training wheel than the area of the regular wheel? *