Find all x such that

a) 2x is congruent to x(mod5)

(When i did this one I there is no such x, but i am not sure)

b) -25<x<25 and x is congrent to 3(mod 5)

(I get confused on the negative side, I know on the positive side you have 3, 8, 13, 18, 23 but how would it go for the negative side, would it be -3, -8, -13, -18, -23 or am i doing this wrong?)