To solve this equation, we first need to isolate the variable x by performing the necessary operations to both sides of the equation:

(8x + 91)/7 = 21

First, multiply both sides by 7 to get rid of the denominator:

8x + 91 = 147

Next, subtract 91 from both sides to isolate the variable x:

8x = 56

Now, divide both sides by 8 to solve for x:

x = 56/8
x = 7

Therefore, the solution to the equation is x = 7.