The formula for the perimeter of a rectangle is. . .
P = 2 * L + 2 * W
where P is perimeter, L is length, and W is width.
Your problem requires us to solve for L, given P and W.
We can first rearrange the formula to solve for L algebraically. . .
(P - 2 * W) / 2 = L
Then, filling the variables in with your given values, we can solve for L. . .
L = (346 - 2 * 76) / 2 = (346 - 152) / 2 = 194 / 2 = 97
The length is 97m.