Write, read, and evaluate expressions in which letters stand for numbers.
6.EE.2.a Write expressions that record operations with numbers and with letters standing for numbers.
6.EE.2.c Evaluate expressions at specific values of their variables. Include expressions that arise from formulas used in real-world problems. Perform arithmetic operations, including those involving whole-number exponents, in the conventional order when there are no parentheses to specify a particular order (Order of Operations).