Defined integral, but one limit needs to be found 2

[AnaAnt]

Hi guys!
Need a little help couse Mathcad doesnt allow me to deine the integral limit as the unknown.
Simple egsample:

integral of "x dx" with limits "a=0 and b=5".
The solution is 12.5
How to reverse the situation so that a=0, b=unknown and provide the previous result of 12.5?
I want Mathcad to find the b value.
How to do this?
Thanx

 

[shinerttu]

Occupant/fel3 thanks!