When three planes intersect in a line but in one of the planes there r two unknowns like

P3: 5x + ?y +17z = u in which u and ? Are unknows while other two eqns of olanes r
P2: 2x-5y+3z=3
P1:3x+2y-5z=-5
So how do u find the unknows given that these three planes intersect in a line

Pleaseee help your help will be appreciated

The way it is written would be difficult to solve. Say you had a value for u or ? In P3, then you could find where P1 and P2 intersect and use that to solve for P3. That's the general idea anyway.