The most commonly used method to find the value of algebraic expression is method of substitution, that is, the numerical value represented by letters is directly substituted for calculation and evaluation.

Sometimes, instead of giving the specific values of letters, we simply simplify and find the values of letters. Sometimes given the relationship of several letters, we can simplify the algebraic expression into a formula with only one letter and then substitute it.

for instance

What if 3a? -a-2=0, so 5+2a-6a? =

Analysis: From 3a? -a-2=0, so -2 =-3a? +a

Multiply both sides of the equation by 2 to get -4 =-6a? +2a

Put 2a-6a? The whole is equal to -4, and the whole is substituted into 5+2a-6a? = 1