8b-12 for b =2.1
we have to evaluate it and idk how

To evaluate this situation, you need to look at the given information. We know that b is 2.1 and that the expression b is put into is 8b-12. So, just plug in for b. 
8*2.1 - 12. Use the order of operations to ensure a correct answer. So, multiply first. 8 * 2.1 is 16.8. Nou, subtract. 16.8 - 12 = 4.8. So, the final answer is 4.8