You may want to review equations first.
Equations of lines are like the equations you're used to, except for one thing: they can have both x and y in them. That means that, for you to check an equation, we need to give you two numbers to check (one for x and one for y), not just one. (The idea is that, if the pair of numbers checks, then if you plot the point on graph paper it will be on the graph of the equation. If you didn't understand that, though, you can still do these problems.)
Your teacher will probably give you the pair of numbers to check as an ordered pair. That means it will look like (−1, 3), which means that x is −1 and y is 3. If you're asked whether (−1, 3) lies on the line y = x + 3, then you just put −1 in where you see x and 3 where you see y. This gives you 3 = −1 + 3. Since −1 + 3 = 2, this is wrong, so the answer is no. If you were asked about (2, 5), then substituting them in would give you 5 = 2 + 3, which is right, so the answer would be yes.
It doesn't matter if the two numbers in the ordered pair are the same or different. If they're the same, that just means you put the same number in for x as for y.
It's also okay if the equation of the line doesn't actually have one of the letters in it. In that case, that number in the ordered pair doesn't matter.