A is a 2-digit square number. In order to find the smallest N number we want to find the smallest possible 2-digit sqaure number. To sqaure a number means to multiply this number by itself, e.g. the square number of 2 is 4 as 2x2=4. Instead of writing 2x2 we write 22. Hence, 1 squared equals 1 (12=1); 22=4; 32=9; 42=16, therefore the smallest square number is 16, so A=16
B is a 2-digit cube number. Again, in order to find the smallest N number we want to find the smallest possible 2 -digit cube number. To find the cube number, we multiply a given number by itself 3 times, e.g. the cube bumber of 2 is 8 as 2x2x2=8. If we follow the same logic, 1 cubed equals 1 (13=1); 23=8; 33=27, therefore the smallest cubed number is 27, so B=27.
N=2A + B; if we substitue A with 16 and B with 27, we get N=2x16 + 27, therefore N=59