There is no reason to let this scare you.

You already know how to solve a linear equation.

2y-x-6=0

x = 2y - 6

You already now how to substitute.

x^2+4y^2=36

(2y - 6)^2+4y^2=36

Expanding and Simplifying are not new

4y^2 - 24y + 36 + 4y^2 = 36

8y^2 - 24y + 36 = 36

8y^2 - 24y = 0

y^2 - 3y = 0

If all else fails, we could here resort to the Quadratic Formula. In this case, since we lost the constant term, it factors easily.

y(y - 3) = 0

y = 0 or y = 3

My point here, is that you should not consider every problem you have not seen as a brand new experience. You have the tools. Use them.