Well, let's write it out, shall we?
6c2-5d+8
Now plug in the numbers.
6(5^2)2-5(4)+8
Now simplify using PEMDAS
Parentheses
Exponents
Multiplication
Division
Addition and Subtraction (They go together go from left to right in the equation)
5^2= 5*5 = 25
6 * 25 = 150
150 * 2 = 300
5 * 4 = 20
Rewrite the equation.
150-20+8
Use PEMDAS again ( refer to the previous mentioning)
150 - 20 = 130
130 + 8 = 138