Here is the way I would do it.

y=2x+b

30=2(40)+b

30=80+b

-50=b

y=2x-50

then y=6x+b

30=6(40)+b

30=240+b

-210=b

y=6x-210

we have y=2x-5 and y=6x-210

0=2x-5 and 0=6x-210

5=2x 210=6x

x=5/2 x=35

total answer of 32.5 but it is supposed to be 10 what am i doing wrong?