I have a problem on 2-dimensional affine cipher, please help me.

R=F16, n=2, A=(4 8)(5 f) , b=(4,3)

A is a matrix with two columns, two rows.

K=(A,b)

eK(7,c)=?

The result is (1,3).

I don't know how to compute eK(7,c) as above. Please help me. I am looking forward to your reply.