Solving the congruence $7x + 3 = 1 \mod 31$? I am having a problem when the LHS has an addition function; if the question is just a multiple of $x$ it's fine.
But when I have questions like $3x+3$ or $4x+7$, I don't seem to get the right answer at the end.

$7x+3\equiv1\implies7x\equiv1-3=-2\equiv29\equiv60\equiv91\implies x\equiv13\pmod {31}.$
