Solve for n in the literal equation a(n−4)+3=bn.

Distribute a
an-4a+3=bn
minus bn from both sides
an-bn-4a+3=0
add 4a to both sides
an-bn+3=4a
minus 3 from both sides
an-bn=4a-3
undistribute n from left side
n(a-b)=4a-3
divide both sides by (a-b)

Equation for length:
L=3w-35

By inputting this into the equation to find the perimeter we get:
2(3w-35)+2w=190
Distribute
6w-70+2w=190
Add like terms
8w=260
Divide both sides by 8
w=32.5
The width is 32.5cm

To find the length input w into the equation for length
L=3(32.5)-35
L=97.5-35
L=62.5
The length is 62.5cm

Check that the solution is correct by inputting the solutions to the perimeter equation

2L+2w=190
2(62.5)+2(32.5)=190
125+65=190
190=190

I hope this helps :)