Similar to Goalseek

I would like to find out how to make a function work that is similar to goalseek. I need to have B11 equal to C1 by changing cell D8. The function is listed in cell B11. At the same time, I need B12 to equal I1 by changing D9. I would like for this process to update after I change the values in C1 and I1.

Any suggestions on code?