On the first one, you are on the right track, but one thing. You found the y-coordinate of c, which is f(c). They want to know the x-coordinate, c. So, set sin(x)=f(c) (the value you got) and solve for c.
On the second one, you know that sin(x)+ln(y) must equal some constant correct? So set it up like this.
See how that works? I'm sure you can finish it from there.