What is the system of measurement used most in the united states?

U.S customary system also known as the English system.

Founded by the British, but they dropped it. Now only three countries in the world still use this system.

step-by-step explanation:

ok so im not 100% sure but you would start by adding 5.1t and 1.9t beacuse they have the same variable. that is 7t.   then add -3.6 + 1.2 which is -2.4

so then keep that as -2.4 + 7t. i apologize if this is incorrect. best luck on your test < 3