For $x > 0$, the area of the triangle with vertices $(0, 0), (x, 0)$ and $(x, 5)$ is 30 square units. What is the value of $x$?
Sketching the points, we find that the triangle is a right triangle whose legs measure $x$ and $5$ units.  Solving $\frac{1}{2}(x)(5)=30$, we find $x=\boxed{12}$.

[asy]
size(5cm,IgnoreAspect);
import graph;
defaultpen(linewidth(0.7)+fontsize(10));
real x = 12;
pair A=(0,0), B=(x,0), C=(x,5);
pair[] dots = {A,B,C};
dot(dots);
draw(A--B--C--cycle);
xaxis(-2,14,Arrows(4));
yaxis(-1,7,Arrows(4));
label("$(0,0)$",A,SW);
label("$(x,0)$",B,S);
label("$(x,5)$",C,N);[/asy]