The pathway to the text edit box may vary depending on the complexity of your report. The code below works for that particular report and probably for many other simple bivariate reports. Use the tree structure display for your particular report to find out the relative (or absolute) pathway.
dt = Open("$SAMPLE_DATA/Big Class.jmp");
biv = dt << Bivariate(Y(:weight), X(:height), Fit Line());
rbiv=report(biv);
// Two equivalent ways to address the text box with the equation
eq1=rbiv[Outline Box(2)][1, 1] << get text; // "First item of the first item of the second outline box"
eq2=rbiv[Text Edit Box(4)] << get text; // Direct address. Numbering not always predictable.
show(eq1,eq2);
// Explore the tree structure to detrmine the pathway to the displaybox of interest
rbiv<<show tree structure;