Let the quantities be x and y.
Graphs tell exactly (or at least approximately) the value/s of x for a certain known y,
or the opposite, assume we want to find y for x=2.5.
We check the graph on the point with x=2.5, and we can see what y is.
check the picture of the graph of y=-3x-xe^2x, generated by desmos.com
assume we want to know what is the value of x for which y=5.
if we click on the point with y-coordinate =5, we see that x=-1.67.
But even without doing this, we can tell that x is a value between x=-2 and x=-1, almost in the middle, but a little more near x=-2.
So we could give an approximation of -1.6 or -1.7, which may be good enough.
An equation also tells what y is, for a given x, or the other way around.
Sometime substituting x with a known value may give an expression in terms of y which is hard to solve,
but for that particular x, the expression in terms of y is uniques, even if a certain value is not produced.
Consider again the equation y=-3x-xe^2x,
for x=2, we can tell that y=-6-2e^4
for y=8, we can say that
8=-3x-xe^2x, and really not be able to solve it, but we can approximate x by checking some values, which is still better that having no clue at all.
