1) Try plugging in the values you have already been given into the equation L=mT+b.
2) After plugging this in, you should get 9982=m(5)+b and 10016=m(34)+b. (Remember, L equals the millimeters in this equation, and T equals the temperature.)
3) Now we use substitution to solve the rest. With the equation 9982=m(5)+b, we can change this to 9982/m(5)=b.
4) Now that we know the value for be, we can insert this into the second equation, in replacement of b: 10016=m(34)+(9982/m(5)) = 34/29. Now we know that the value m equals 34/29.
5) Replace the m values in the first equation with your newfound value: 9982=(34/29)(5)+b = 289308/29.
