Using k(concrete)=1 W/mK & k(fiberglass)=0.042 W/mK , you can use the Heat Flow equation: H=kA(ΔT/Δx)
It may help to make up imaginary values for A and ΔT
Also note that 3.5 inches = 0.0889 meters
Solve for H(fiberglass) by using the above equation and values. Set any values for A and ΔT...