Answer:
a.) (x-3)(x-3) = 25 + (x-4)(x-4)
x^2 - 6x + 9 = 25 + x^2 - 8x + 16
-6x + 9 = -8x + 41
2x + 9 = 41
2x = 32
x = 16
b.) 16 - 4 = 12
AB = 12
c.) 16 - 3 = 13
AC = 13
[btw I don't know if this is the method your teacher wants as it does not use the similar triangle :/ but it is still correct :) so good luck bro]