Can someone explain substitution more clearly?

As in solving systems by substitution. I'm genuinely stumped, can the process be explained in a coherent and straightforward manner with examples?

Ms. Johnson asked her class to write an equivalent expression to 14d - 9 + 21k - 7dk + 2 • Riley wrote the expression 7dk (2 - 3 - 1) - 7 • Ahmad wrote the expression 7 (2d - 3k - dk) - 7 • Ariana wrote the expression 7k (2d - 3 - d) - 7 Who us

20*^2-30x-40 solve for x

A tank holds 1,000 gallons of syrup. Danielle filled 65 5-gallon cans with syrup. How much syrup was left in the tank when she finished?

Daria's goals is to run 7 miles each weekend. she ran 3 1/3 miles on Saturday and 3 3/4 miles on Sunday. By how much did she beat her goal

Mary bakes a chocolate cake, a vanilla cake, and a strawberry cake. Each cake is the same size. Angela eats four-eighths of the chocolate cake, two-eighths of the vanilla cake, and two-fourths of the strawberry cake.

126 pieces of white chocolate and 84 pieces of dark chocolate are divided into packs with equal number of pieces of each kind of chocolate. what is the greatest possible number of packs needed?

If beth washes dishes every day for 14 days how much will she money will she earn

a rectangular prism with a volume of 3 cubic units is filled with cubes with side lengths of 1/4 unit. how many 1/4 unit cubes does it take to fill the prism

Work out the surface area of a cube of edge 3m

If the radius of a flying disc is 7.6 centimeters, what is the approximate area of the disc? 23.864 square centimeters