In both cases, solve the second equation for one variable in terms of the other, and then substitute into the first. For example, with:
we can solve the second for y:
and substituting into the first equation:
so x=3 or x=-4, and substituting into the second equation, we get y=4 or y=-3 respectively, so we have (3,4) and (-4,-3) as solutions.
The second can be solved similarly.