Area is length times width in this scenario. The length is going to increase by 3; this can be expressed by the term L+3. The width is 5, but the total area is going to be 90. Your expression should look like this:
5(L+3)=90
First, multiply the 5 on the left side of the equation.
5L+15=90
Subtract 15 from both sides to get the coefficient and the variable by itself.
5L=75
Divide by 5 on both sides to get the variable alone.
L=15
You're not done yet. Remember, you must add 3 back into L to get the new length. The original garden would have been 15 meters in length, but the new length of the new garden will be 18 meters.
To check this, use the formula for area.
18(5)=90
90=90